[STATEMENT]
lemma OclAnd15[simp]: "(true and null) = null"
[PROOF STATE]
proof (prove)
goal (1 subgoal):
 1. (true and null) = null
[PROOF STEP]
by(rule ext,simp add: OclAnd_def null_def invalid_def true_def false_def bot_option_def
                        null_fun_def null_option_def)