Functionalities provided by the language can be overwritten by simply defining a new one.
None of the "core functions", "reference functions" or "object functions" can be overwritten (although many of them have alternatives which can indeed be overwritten). No basic language constructs can be overwritten either.
Where it can be achieved, it is done by simply declaring the function within the code.
In this example, we are overwriting the
multiply (removed as of version 1.3.7)
function multiply($n, $times) //Multiply $out = 0 for($i = 0, $i < $times, 1) $out = $out + $n end for return $out end function
Here is another example:
function input_validate($x, $pattern) return string_matches($x, $pattern) end function