Arguments which are omitted are passed to the function as nil. In addition to positional arguments, a function (invoked by either of the above two mechanisms). Keyword arguments look like this:
f,arg1, keyA= exprA, keyB= exprB, arg2, ...
where keyA and keyB are the names of keyword arguments of the function f. Omitted keywords are passed to f as nil values. Keywords typically set optional values which have defaults.