6.14 Implement a function VALIDITY that takes a sentence as input and returns either valid,

—^ " satisfiable, or unsatisfiable. Use it to answer the questions in Exercise 6.12. You will need to define an implementation-level representation of sentences. The cleanest way to do this is to define an abstract data type for compound sentences. Begin by writing EVAL-TRUTH as a recursive function that takes a sentence and an assignment of truth values to proposition symbols, and returns true or false. Then call EVAL-TRUTH for all possible assignments of truth values to the proposition symbols.