A _strategy definition_ f(x1,...,xn) = s defines a strategy operator =f= with =n= strategy parameters. An application =f(s1,...sn)= of this operator is equivalent to let x1 = s1 ... xn = sn in s that is, binding the actual strategy parameters =si= to the formal parameters =xi=. Strategy definitions may be recursive. The RecursionOperator =rec= can be used to create recursive strategy locally without need to give a name. rec x(s) is equivalent to let x = s in x end Typical examples of strategy definitions are try(s) = s <+ id repeat(s) = try(s; repeat(s)) topdown(s) = s; all(topdown(s))