Normally, mappings are extended by simply inserting new elements. This function is useful if you can estimate the needed size of the mapping in advance, to reduce the malloc overhead. Any unused allocated space will be freed after the current function returns.
mkmapping( ({ 1, 2 }), ({ 10, 11 }), ({ 20, 21, 22}))
returns ([ 1:10;20, 2:11;21 ])