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Ascii Art

It is possible to generate Sequence Diagrams using ASCII Art.
@startuml
participant Bob
actor Alice
 
Bob -> Alice : hello
Alice -> Bob : Is it ok?
@enduml
You have to use the -txt flag in the command line, or the format tag in the ANT task :
  <target name="main">
    <plantuml dir="./src" format="txt" />
  </target>
Generated files use .atxt extension. ("a" as ASCII art : to NOT overide any existing .txt files!). Here is the result:
                      ,-.  
                      `-'  
                      /|\  
     ,---.             |   
     |Bob|            / \  
     `-+-'           Alice 
       |    hello      |   
       |-------------->|   
       |               |   
       |  Is it ok?    |   
       |<- - - - - - - |   
     ,-+-.           Alice 
     |Bob|            ,-.  
     `---'            `-'  
                      /|\  
                       |   
                      / \  

Unicode

The default txt format uses only plain ASCII characters. It is possible to use few extended Unicode characters to have a slightly better result. You should use the -utxt flag in the command line, or the utxt format in the ANT task.
                      ┌─┐  
                      ║"│  
                      └┬┘  
                      ┌┼┐  
     ┌───┐             │   
     │Bob│            ┌┴┐  
     └─┬─┘           Alice 
       │    hello      │   
       │──────────────>│   
       │               │   
       │  Is it ok?    │   
       │<─ ─ ─ ─ ─ ─ ─ │   
     ┌─┴─┐           Alice 
     │Bob│            ┌─┐  
     └───┘            ║"│  
                      └┬┘  
                      ┌┼┐  
                       │   
                      ┌┴┐  
They are two drawbacks of using utxt:

Complexe Diagram

You can even try complex example if you wish.
@startuml
'hide footbox

participant "Bob on\nseveral lines" as Bob
actor Alice

Bob -> Alice : hello
note right of Alice
  this is a note
end note

Alice -> Bob : Is it ok\nwith a message that is\non several lines?

note right
  This other note
  should work
  on several lines
end note

== This is a separation ==

Bob -> Last : Yes it works!
Last -> Last : working in progress
note left : this is\nanother note

Last --> Last : working in progress

Last --> Bob : done

opt dummy comment
  Bob -> Last : Error\nOn\nSeveral\nLine
  Last --> Bob : None
else
  Last --> Bob : None
  Last -> Bob : None
else other
  Last -> Bob : None
  note over Alice, Last
    This is a long note
    over Alice and Last
  end note
  Last -> Bob : None
  Last -> Bob : None
end


@enduml
		

Example

                                             ,-.                                                   
                                             `-'                                                   
               ,-------------.               /|\                                                   
               |Bob on       |                |             ,----.                                 
               |several lines|               / \            |Last|                                 
               `------+------'              Alice           `-+--'                                 
                      |        hello          |               |                                    
                      |---------------------->|               |                                    
                      |                       |               |                                    
                      |                       | ,--------------!.                                  
                      |                       | |this is a note|_\                                 
                      |                       | `----------------'                                 
                      |Is it ok               | ,----------------!.                                
                      |with a message that is | |This other note |_\                               
                      |on several lines?      | |should work       |                               
                      |<----------------------| |on several lines  |                               
                      |                       | `------------------'                               
                      |                       |               |                                    
                      |              ======================== |                                    
====================================== This is a separation =======================================
                      |              ======================== |                                    
                      |                       |               |                                    
                      |            Yes it works!              |                                    
                      |-------------------------------------->|                                    
                      |                       |               |                                    
                      |                       ,------------!. |----.                               
                      |                       |this is     |_\|    | working in progress           
                      |                       |another note  ||<---'                               
                      |                       `--------------'|                                    
                      |                       |               |- - .                               
                      |                       |               |    | working in progress           
                      |                       |               |< - '                               
                      |                       |               |                                    
                      |                 done  |               |                                    
                      |<- - - - - - - - - - - - - - - - - - - |                                    
                      |                       |               |                                    
     ______________________________________________________________________                        
     ! OPT  /  dummy comment                  |               |            !                       
     !_____/          |                       |               |            !                       
     !                |                       |               |            !                       
     !                |               Error   |               |            !                       
     !                |               On      |               |            !                       
     !                |               Several |               |            !                       
     !                |               Line    |               |            !                       
     !                |-------------------------------------->|            !                       
     !                |                       |               |            !                       
     !                |                 None  |               |            !                       
     !                |<- - - - - - - - - - - - - - - - - - - |            !                       
     !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!                       
     !                |                       |               |            !                       
     !                |                 None  |               |            !                       
     !                |<- - - - - - - - - - - - - - - - - - - |            !                       
     !                |                       |               |            !                       
     !                |                 None  |               |            !                       
     !                |<--------------------------------------|            !                       
     !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!                       
     ! [other]        |                       |               |            !                       
     !                |                 None  |               |            !                       
     !                |<--------------------------------------|            !                       
     !                |                       |               |            !                       
     !                |                    ,-------------------!.          !                       
     !                |                    |This is a long note|_\         !                       
     !                |                    |over Alice and Last  |         !                       
     !                |                    `---------------------'         !                       
     !                |                 None  |               |            !                       
     !                |<--------------------------------------|            !                       
     !                |                       |               |            !                       
     !                |                 None  |               |            !                       
     !                |<--------------------------------------|            !                       
     !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!                       
               ,------+------.              Alice           ,-+--.                                 
               |Bob on       |               ,-.            |Last|                                 
               |several lines|               `-'            `----'                                 
               `-------------'               /|\                                                   
                                              |                                                    
                                             / \                                                   

Same example using Unicode

                                             ┌─┐                                                   
                                             ║"│                                                   
                                             └┬┘                                                   
               ┌─────────────┐               ┌┼┐                                                   
               │Bob on       │                │             ┌────┐                                 
               │several lines│               ┌┴┐            │Last│                                 
               └──────┬──────┘              Alice           └─┬──┘                                 
                      │        hello          │               │                                    
                      │──────────────────────>│               │                                    
                      │                       │               │                                    
                      │                       │ ╔═════════════╧══╗                                 
                      │                       │ ║this is a note ░║                                 
                      │                       │ ╚═════════════╤══╝                                 
                      │Is it ok               │ ╔═════════════╧════╗                               
                      │with a message that is │ ║This other note  ░║                               
                      │on several lines?      │ ║should work       ║                               
                      │<──────────────────────│ ║on several lines  ║                               
                      │                       │ ╚═════════════╤════╝                               
                      │                       │               │                                    
                      │              ╔════════╧═════════════╗ │                                    
══════════════════════╪══════════════╣ This is a separation ╠═╪════════════════════════════════════
                      │              ╚════════╤═════════════╝ │                                    
                      │                       │               │                                    
                      │            Yes it works!              │                                    
                      │──────────────────────────────────────>│                                    
                      │                       │               │                                    
                      │                       ╔══════════════╗│────┐                               
                      │                       ║this is      ░║│    │ working in progress           
                      │                       ║another note  ║│<───┘                               
                      │                       ╚══════════════╝│                                    
                      │                       │               │─ ─ ┐                               
                      │                       │               │    | working in progress           
                      │                       │               │< ─ ┘                               
                      │                       │               │                                    
                      │                 done  │               │                                    
                      │<─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ │                                    
                      │                       │               │                                    
     ╔══════╤═════════╪═══════════════════════╪═══════════════╪════════════╗                       
     ║ OPT  │  dummy comment                  │               │            ║                       
     ╟──────┘         │                       │               │            ║                       
     ║                │                       │               │            ║                       
     ║                │               Error   │               │            ║                       
     ║                │               On      │               │            ║                       
     ║                │               Several │               │            ║                       
     ║                │               Line    │               │            ║                       
     ║                │──────────────────────────────────────>│            ║                       
     ║                │                       │               │            ║                       
     ║                │                 None  │               │            ║                       
     ║                │<─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ │            ║                       
     ╠════════════════╪═══════════════════════╪═══════════════╪════════════╣                       
     ║                │                       │               │            ║                       
     ║                │                 None  │               │            ║                       
     ║                │<─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ │            ║                       
     ║                │                       │               │            ║                       
     ║                │                 None  │               │            ║                       
     ║                │<──────────────────────────────────────│            ║                       
     ╠════════════════╪═══════════════════════╪═══════════════╪════════════╣                       
     ║ [other]        │                       │               │            ║                       
     ║                │                 None  │               │            ║                       
     ║                │<──────────────────────────────────────│            ║                       
     ║                │                       │               │            ║                       
     ║                │                    ╔══╧═══════════════╧══╗         ║                       
     ║                │                    ║This is a long note ░║         ║                       
     ║                │                    ║over Alice and Last  ║         ║                       
     ║                │                    ╚══╤═══════════════╤══╝         ║                       
     ║                │                 None  │               │            ║                       
     ║                │<──────────────────────────────────────│            ║                       
     ║                │                       │               │            ║                       
     ║                │                 None  │               │            ║                       
     ║                │<──────────────────────────────────────│            ║                       
     ╚════════════════╪═══════════════════════╪═══════════════╪════════════╝                       
               ┌──────┴──────┐              Alice           ┌─┴──┐                                 
               │Bob on       │               ┌─┐            │Last│                                 
               │several lines│               ║"│            └────┘                                 
               └─────────────┘               └┬┘                                                   
                                             ┌┼┐                                                   
                                              │                                                    
                                             ┌┴┐                                                   

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