Theorem ssun4 3430

Description: Subclass law for union of classes. (Contributed by NM, 14-Aug-1994.)

Assertion

Ref	Expression
ssun4	⊢ (A ⊆ B → A ⊆ (C ∪ B))

Proof of Theorem ssun4

Step	Hyp	Ref	Expression
1		ssun2 3428	. 2 ⊢ B ⊆ (C ∪ B)
2		sstr2 3280	. 2 ⊢ (A ⊆ B → (B ⊆ (C ∪ B) → A ⊆ (C ∪ B)))
3	1, 2	mpi 16	1 ⊢ (A ⊆ B → A ⊆ (C ∪ B))

Syntax hints:   → wi 4   ∪ cun 3208   ⊆ wss 3258

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1546  ax-5 1557  ax-17 1616  ax-9 1654  ax-8 1675  ax-6 1729  ax-7 1734  ax-11 1746  ax-12 1925  ax-ext 2334

This theorem depends on definitions:  df-bi 177  df-or 359  df-an 360  df-nan 1288  df-tru 1319  df-ex 1542  df-nf 1545  df-sb 1649  df-clab 2340  df-cleq 2346  df-clel 2349  df-nfc 2479  df-v 2862  df-nin 3212  df-compl 3213  df-in 3214  df-un 3215  df-ss 3260

This theorem is referenced by:  ssun  3443