Constructs a dyn-tuple 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
tuple, 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.