[STATEMENT]
lemma union_le_diff_plus: "P \<subseteq># M \<Longrightarrow> N < P \<Longrightarrow> M - P + N < M"
[PROOF STATE]
proof (prove)
goal (1 subgoal):
 1. \<lbrakk>P \<subseteq># M; N < P\<rbrakk> \<Longrightarrow> M - P + N < M
[PROOF STEP]
by (drule subset_mset.diff_add[symmetric]) (metis union_le_mono2)