 Solution

For example, takeThis is a decidable language: check that the input is a valid Turing machine encoding, count its declared states, and test the count's integer parity. There are Turing machines with even state counts and Turing machines with odd state counts, so the formal language contains some but not all valid descriptions.

There is no contradiction with Rice's theorem because this is a syntactic property of the description. Add one unreachable state to a Turing machine, assigning any permitted transitions to it. The accepted formal language is unchanged, while the integer parity of the state count changes. Thus the property is not an extensional property of programs, which is an essential hypothesis of Rice's theorem.