[STATEMENT]
lemma list_len_1_is_singleton: "length l = 1 \<Longrightarrow> (\<exists>x. l = [x])"
[PROOF STATE]
proof (prove)
goal (1 subgoal):
 1. length l = 1 \<Longrightarrow> \<exists>x. l = [x]
[PROOF STEP]
by (induct l) auto