[STATEMENT]
lemma or_lem: "(a=c) ==> (b=d) ==> (a | b) = (c | d)"
[PROOF STATE]
proof (prove)
goal (1 subgoal):
 1. \<lbrakk>a = c; b = d\<rbrakk> \<Longrightarrow> (a \<or> b) = (c \<or> d)
[PROOF STEP]
by simp