A = {p | p(x)↑ whenever the first letter of x
is 'a'} is not computable by Rice's theorem. A is nontrivial because
some programs halt on every string that starts with 'a', and some
programs don't do that. A respects equivalence because,
if p and q are two equivalent programs then either p and q both
halt of every string that begins with 'a' or both do not have that property.