parse_as_prefix : string -> unit

SYNOPSIS: Gives an identifier prefix status.
DESCRIPTION: Certain identifiers c have prefix status, meaning that combinations of the form c f x will be parsed as c (f x) rather than the usual (c f) x. The call parse_as_prefix "c" adds c to the list of such identifiers.
FAILURE CONDITIONS: Never fails, even if the string already has prefix status.
SEE ALSO: is_prefix, prefixes, unparse_as_prefix.