Theorem ssun4 1624

Description: Subclass law for union of classes.

Assertion

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

Proof of Theorem ssun4

Step	Hyp	Ref	Expression
1		ssun2 1622	. 2 ⊢ B ⊆ (C ∪ B)
2		sstr2 1510	. 2 ⊢ (A ⊆ B → (B ⊆ (C ∪ B) → A ⊆ (C ∪ B)))
3	1, 2	mpi 44	1 ⊢ (A ⊆ B → A ⊆ (C ∪ B))

Syntax hints:   → wi 2   ∪ cun 1485   ⊆ wss 1487

This theorem is referenced by:  ssun 1634  xpex 2488

This theorem was proved from axioms:  ax-1 3  ax-2 4  ax-3 5  ax-mp 6  ax-4 673  ax-5 674  ax-6 675  ax-7 676  ax-gen 677  ax-8 798  ax-9 799  ax-10 800  ax-11 801  ax-12 802  ax-16 922  ax-17 925  ax-ext 1074

This theorem depends on definitions:  df-bi 128  df-or 197  df-an 198  df-ex 679  df-sb 853  df-clab 1093  df-cleq 1097  df-clel 1099  df-v 1349  df-un 1490  df-in 1491  df-ss 1492

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