Rules can call out to other peg.ebnf defined parsers. The result of the foreign call then becomes the AST of the successful parse. Foreign rules are invoked using '<foreign word-name>' or '<foreign word-name rule>'. The latter allows calling a specific rule in a previously designed peg.ebnf parser. If the 'word-name' is not the name of a peg.ebnf defined parser then it must be a word with stack effect ( -- parser ). It must return a peg defined parser and it will be called to perform the parse for that rule.