 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;
 reserve L for Nelson_Algebra,
         a, b, c, d, x, y, z for Element of L;

theorem Th25: :: (2.26)
  ((a => (b => c)) => ((a => b) => (a => c))) = Top L
  proof
    a "/\" (a => b) < b by Th17; then
    a "/\" (a => b) "/\" (a => (b => c)) < b "/\" (a => (b => c))
      by Lm1; then
    a "/\" (a => b) "/\" (a => (b => c)) < b "/\" (b => (a => c))
      by Th18; then
A1: a "/\" (a => b) "/\" (b => (a => c)) < b "/\" (b => (a => c)) by Th18;
A2: b "/\" (b => (a => c)) < a => c by Th17;
    a "/\" (a => b) "/\" (b => (a => c)) < a => c by Def3,A1,A2; then
    a "/\" (a "/\" (a => b) "/\" (b => (a => c))) < c by Def4; then
    a "/\" (a "/\" ((a => b) "/\" (b => (a => c)))) < c by LATTICES:def 7; then
    (a "/\" a) "/\" ((a => b) "/\" (b => (a => c))) < c by LATTICES:def 7; then
    a "/\" ((a => b) "/\" (a => (b => c))) < c by Th18; then
    (a => b) "/\" (a => (b => c)) < (a => c) by Def4; then
    (a => (b => c)) < ((a => b) => (a => c)) by Def4;
    hence thesis;
  end;
