int m_contains(mixed &data1, ..., &dataN, map, key)
If the mapping contains the key map, the corresponding values
are assigned to the data arguments, which must be passed by
reference, and 1 is returned. If key is not in map, 0 is
returned and the data args are left unchanged.
It is possible to use this function for a 0-value mapping, in
which case it has the same effect as member(E).
Renamed from 'mapping_contains()' in LDMud 3.2.6.
m_entry(E), mappings(LPC), member(E)