package lambdapi

is_escaped s tells if s begins with "{|" and ends with "|}" without overlapping. For efficiency, we just test that it starts with '{'.

unescape s removes "{|" and "|}" if s is an escaped identifier.

p is assumed to be a regular identifier. If n is a regular identifier too, then add_prefix p n is just p ^ n. Otherwise, it is "{|" ^ p ^ unescape n ^ "|}".

s is assumed to be a regular identifier. If n is a regular identifier too, then add_suffix n s is just n ^ s. Otherwise, it is "{" ^ unescape n ^ s ^ "|}".