try_user_parser : lexcode list -> preterm * lexcode list

SYNOPSIS: Try all user parsing functions.
DESCRIPTION: HOL Light allows user parsing functions to be installed, and will try them on all terms during parsing before the usual parsers. The call try_user_parser l attempts to parse the list of tokens l using all the user parsers, taking the results from whichever one succeeds first.
FAILURE CONDITIONS: Fails if all user parsers fail.
SEE ALSO: delete_parser, install_parser, installed_parsers.