There are many ways to try to make the ANTLR code simpler.

One approach is to break up rules into smaller rules.  It's easier to *implement*
some of these rules as a long sequence of conditions, but if that presentation
makes it too hard to explain, then we can of course change it.
Also, we could remove some of the "greedy=true" statements (which are used
to eliminate some warning messages).

Below is a first shot at breaking up it_expr, currently 1 long rule, into 2 rules.
This could obviously be repeated to make more rules, each one simpler.
Not saying it's done, but would it help to break the current longer rules
into more but smaller rules?

--- David A. Wheeler

it_expr_normal returns [Object v]
  : head
     GROUP_SPLIT hspace*
      (comment_eol error
       | /*empty*/ {(monify $head)} )
     | SUBLIST hspace*
       (sub_i=it_expr {(append $head (list $sub_i))}
        | comment_eol error )
     | comment_eol // Normal case, handle child lines if any:
       (INDENT children=body {(append $head $children)}
        | /*empty*/          {(monify $head)} /* No child lines */ )
    ;

it_expr_prefixed returns [Object v]
  : (GROUP_SPLIT | scomment) hspace* /* Initial; Interpet as group */
      (group_i=it_expr {$group_i} /* Ignore initial GROUP/scomment */
       | comment_eol
         (INDENT g_body=body {$g_body} /* Normal GROUP use */
          | same ( g_i=it_expr {$g_i} /* Plausible separator */
                   /* Handle #!sweet EOL EOL t_expr */
                   | comment_eol restart=t_expr {$restart} )
          | DEDENT error ))
  | SUBLIST hspace* /* "$" first on line */
    (is_i=it_expr {(list $is_i)}
     | comment_eol error )
  | abbrevw hspace*
      (comment_eol INDENT ab=body
         {(append (list $abbrevw) $ab)}
       | ai=it_expr
         {(list $abbrevw $ai)} ) ;

it_expr returns [Object v]
  : it_expr_normal   {$it_expr_normal}
  | it_expr_prefixed {$it_expr_prefixed} ;