Just as a computer can execute a program, a human, e.g. programmer,
customer, or user, can execute (simulate) the specification of a program,
perhaps as part of validating the specification before bothering to write
code from it. But be careful that the intelligent humans can recognize that
they are thinking, Doing What It Means (DWIM), and not spotting a defect
that will trip up the dumb computer wich Does Only What It Says (DOWIS).
E.g., for x**2+3x+2/y, a human would know that if y were 0, this formula is
meaningless, but a program for the same that does not check that y =/= 0
will fail.