| F_call of string | n-ary function |
| F_subst of string * string | n-ary function with substitution first value is the link name, second is the substitution (e.g. "foo(%1,%2)") |
| F_left of string | 2-ary function left-to-right + |
| F_right of string | 2-ary function right-to-left + |
| F_list of string * string | n-ary function with (cons,nil) constructors |
| F_assoc of string | associative infix operator |
| F_bool_prop of string * string | Has a bool and prop version |