SYNOPSIS
        mapping m_delete(mapping map, mixed key)

DESCRIPTION
        Remove the entry with index 'key' from mapping 'map'. The
        changed mapping 'map' is also returned as result.
        If the mapping does not have an entry with index 'key',
        nothing is changed.

SEE ALSO
        mappingp(E), mkmapping(E), m_add(E), m_indices(E), m_values(E),
        sizeof(E), widthof(E)