prefer_form_with_tok : {term_name : string, tok : string} -> unit
STRUCTURE
Parse
SYNOPSIS
Sets a grammar rule’s preferred flag, causing it to be preferentially printed.
LIBRARY
Parse
DESCRIPTION
A call to prefer_form_with_tok causes the parsing/pretty-printing rule specified by the term_name-tok combination to be the preferred rule for pretty-printing purposes. This change affects the global grammar.
FAILURE
Never fails.
EXAMPLE
The initially preferred rule for conditional expressions causes them to print using the if-then-else syntax. If the user prefers the “traditional” syntax with =>-|, this change can be brought about as follows: 
   - prefer_form_with_tok {term_name = "COND", tok = "=>"};
   > val it = () : unit

   - Term`if p then q else r`;
   <<HOL message: inventing new type variable names: 'a.>>
   > val it = `p => q | r` : term

COMMENTS
As the example above demonstrates, using this function does not affect the parser at all.

There is a companion temp_prefer_form_with_tok function, which has the same effect on the global grammar, but which does not cause this effect to persist when the current theory is exported.

HOL  Kananaskis-8