is_prefix : string -> bool

SYNOPSIS: Tests if an identifier has 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 is_prefix "c" tests if c is one of those identifiers.
FAILURE CONDITIONS: Never fails.
SEE ALSO: parse_as_prefix, prefixes, unparse_as_prefix.