operator: exp

    description:
        exp[op, n] |x>
        essentially the first n terms of exp(op) applied to |x>, ignoring factors of 1/n!
        if you want the 1/n! too, then see full-exp
        cf: exp(A) |Psi> in quantum mechanics
        ie: (1 + op + op^2 + ... + op^n) |x>
        if n <= 0, return |x>
         
    examples:
        -- load a binary tree:
        load tree.sw

        -- find left branch of tree:
        exp[left, 3] |x>
            |x> + |0> + |00> + |000>

        -- find right branch of tree:
        -- NB: in this case exp[right, 4] would have sufficed
        exp[right, 10] |x>
            |x> + |1> + |11> + |111> + |1111>

        -- find the head of the tree:
        exp[child, 2] |x>
            |x> + |0> + |1> + |00> + |01> + |10> + |11>

    see also:
        full-exp, exp-max, sexp
        
    TODO:
        currently it doesn't handle sequences all that well.