Modal is a language based a model of computation that uses pattern matching and replacement to rewrite trees.
All Modal code is represented as a series of rules applied to a given tree, formatted textually as tokens delimited with parenthesis, which gets continually modified until no rules match any given part of the tree.
A pattern/replacement can be:
A token: foo, bar, baz
A variable: ?foo, ?bar, ?baz
A sequence of the above: (foo bar baz), (foo ?bar baz), foo (bar (baz))