u M. is the same as u but may keep a record of the
arguments and results for reuse. It is commonly used
for multiply-recursive verbs.
The following examples illustrate the difference
that memoization can make. fib n is the n-th Fibonacci
number. pn finds the number of partitions of
an integer using a recurrence relation due to Euler, equation 11 in
http://mathworld.wolfram.com/PartitionFunctionP.html .