reserve i for Nat,
  j for Element of NAT,
  X,Y,x,y,z for set;
reserve C for initialized ConstructorSignature,
  s for SortSymbol of C,
  o for OperSymbol of C,
  c for constructor OperSymbol of C;
reserve a,b for expression of C, an_Adj C;
reserve t, t1,t2 for expression of C, a_Type C;
reserve p for FinSequence of QuasiTerms C;
reserve e for expression of C;
reserve a,a9 for expression of C, an_Adj C;
reserve q for pure expression of C, a_Type C,
  A for finite Subset of QuasiAdjs C;
reserve T for quasi-type of C;

theorem Th97:
  variables_in ((ast C)term(a,t)) = (variables_in a)\/(variables_in t)
proof
  (ast C)term(a,t) = [ *, the carrier of C]-tree <*a,t*> by Th46;
  then variables_in ((ast C)term(a,t))
  = ((C variables_in a)(\/)(C variables_in t)).a_Term by Th96;
  hence thesis by PBOOLE:def 4;
end;
