For testing only two values, I'd personally do this:
if x ~= 0 and x ~= 1 then
print( "X must be equal to 1 or 0" )
return
end
If you need to test against more than two values, I'd stuff your choices in a table acting like a set, like so:
choices = {[0]=true, [1]=true, [3]=true, [5]=true, [7]=true, [11]=true}
if not choices[x] then
print("x must be in the first six prime numbers")
return
end