operator: full-exp

    description:
        full-exp[op, n] |x>
        the first n terms of exp(op) applied to |x>, including factors of 1/n!
        if you don't want the 1/n!, then see exp
        cf: exp(A) |Psi> in quantum mechanics
        ie: (1 + op/1! + op^2/2! + ... + op^n/n!) |x>
        if n <= 0, return |x>
        most of the time, you probably just want exp[op, n] or exp-max[op]

    examples:
        -- load a binary tree:
        load tree.sw

        -- find left branch of tree:
        full-exp[left, 3] |x>
            |x> + |0> + 0.5|00> + 0.167|000>            

        -- find right branch of tree:
        -- NB: in this case full-exp[right, 4] would have sufficed
        full-exp[right, 10] |x>
            |x> + |1> + 0.5|11> + 0.167|111> + 0.042|1111>

        -- find the head of the tree:
        full-exp[child, 3] |x>
            |x> + |0> + |1> + 0.5|00> + 0.5|01> + 0.5|10> + 0.5|11> + 0.167|000> + 0.167|001> + 0.167|010> + 0.167|011> + 0.167|100> + 0.167|101> + 0.167|110> + 0.167|111>
            
    see also:
        exp, exp-max