From discussion here: #492 (comment)

Rather than try to keep up with all the various features in PlantUML, we should allow users to instruct StateSmith to ignore everything between special comments.

⚠️ Note! You should be careful with this feature. It is rather powerful. You could abuse it to hide states and transitions from StateSmith (which I wouldn't recommend) 🙃. Best to just hide unsupported styling from StateSmith.

Option A (chosen):

Easiest to parse with confidence. A bit noisy, but also very clear.

' <StateSmith:ignore>
mainframe Some Title
!theme spacelab
' </StateSmith:ignore>

@startuml MySm
[*] -> a
a -> b
@enduml

Possible Antlr4 lexer rule:

SS_IGNORE_COMMENT:
    (START_OF_INPUT | LINE_ENDER)
    HWS*   // optional horizontal whitespace
    SINGLE_QUOTE
    HWS*   // optional horizontal whitespace
    '<StateSmith:ignore>'
    .*?
    '</StateSmith:ignore>'
    -> skip
    ;

Option B (not chosen):

This is less noisy looking, but parsing with confidence is kinda annoying as ANTLR4 can't do lexer lookaheads.

' StateSmith:ignore-begin
mainframe Some Title
!theme spacelab
' StateSmith:ignore-end

@startuml MySm
[*] -> a
a -> b
@enduml

Decision

Go with Option A for now. It is easy to do the "right" thing. We can add support for B if we find A annoying after some use.