.. efun:: mapping m_add(mapping map, mixed key, [mixed data...])

  Add (or replace) an entry with index :arg:`key` in mapping :arg:`map`. The modified mapping is also returned as result.

  The values for the entry are taken from the :arg:`data` arguments. Unassigned entry values default to 0, extraneous :arg:`data` arguments are ignored.

  The difference between :efun:`m_add` and :operator:`+=` is that the latter creates a duplicate of the target mapping if it has several references.

  .. usage::

    ::

      mapping m;

      m = ([ "foo" ]);
      m_add(m, "bar", 1) // ([ "foo", "bar" ])

      m = ([ "foo":1 ]);
      m_add(m, "bar", 1) // ([ "foo":1, "bar":1 ])

      m = ([ "foo":1;2 ]);
      m_add(m, "bar", 1) // ([ "foo":1;2, "bar":1;0 ])
      m_add(m, "baz", ({ 4, 5 })... ) // ([ "foo":1;2, "bar":1;0, "baz":4;5 ])

  :history 3.2.9 introduced:

  .. seealso:: :efun:`mappingp`, :efun:`mkmapping`, :efun:`m_delete`, :efun:`m_entry`, :efun:`m_indices`, :efun:`m_values`, :efun:`sizeof`, :efun:`widthof`