Constructs a map of the default implementation according to the supplied
argument subforms. Each argument subform can be a list of the form (key-expr
value-expr), denoting a mapping from the value of key-expr to the value of
value-expr; or a list of the form ($ expression), in which case the
expression must evaluate to a map, denoting all its mappings. The result is
constructed from the denoted mappings in left-to-right order; so if a given key
is supplied by more than one argument subform, its associated value will be
given by the rightmost such subform.