Theorem sstr2 3255

Description: Transitivity of subclasses. Exercise 5 of [TakeutiZaring] p. 17. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Andrew Salmon, 14-Jun-2011.)

Assertion

Ref	Expression
sstr2	|- ( A C_ B -> ( B C_ C -> A C_ C ) )

Proof of Theorem sstr2

Dummy variable x is distinct from all other variables.

Step	Hyp	Ref	Expression
1		ssel 3242	. . . 4 |- ( A C_ B -> ( x e. A -> x e. B ) )
2	1	imim1d 75	. . 3 |- ( A C_ B -> ( ( x e. B -> x e. C ) -> ( x e. A -> x e. C ) ) )
3	2	alimdv 1932	. 2 |- ( A C_ B -> ( A. x ( x e. B -> x e. C ) -> A. x ( x e. A -> x e. C ) ) )
4		ssalel 3235	. 2 |- ( B C_ C <-> A. x ( x e. B -> x e. C ) )
5		ssalel 3235	. 2 |- ( A C_ C <-> A. x ( x e. A -> x e. C ) )
6	3, 4, 5	3imtr4g 205	1 |- ( A C_ B -> ( B C_ C -> A C_ C ) )

This proof depends on syntax axioms:   -> wi 4   A.wal 1400   e. wcel 2209   C_ wss 3220

This proof depends on axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-5 1500  ax-7 1501  ax-gen 1502  ax-ie1 1546  ax-ie2 1547  ax-8 1557  ax-11 1559  ax-4 1563  ax-17 1579  ax-i9 1583  ax-ial 1587  ax-i5r 1588  ax-ext 2220

This proof depends on definitions:  df-bi 117  df-nf 1514  df-sb 1816  df-clab 2225  df-cleq 2231  df-clel 2234  df-in 3226  df-ss 3233

This theorem is used by:  sstr  3256  sstri  3257  sseq1  3271  sseq2  3272  ssun3  3394  ssun4  3395  ssinss1  3460  ssdisj  3581  sspw  3702  triun  4242  trintssm  4245  sspwb  4356  exss  4367  relss  4862  funss  5396  funimass2  5459  fss  5546  fiintim  7238  sbthlem2  7275  sbthlemi3  7276  sbthlemi6  7279  lsslss  14767  lspss  14785  aspss  15068  tgss  15213  tgcl  15214  tgss3  15228  clsss  15268  neiss  15300  ssnei2  15307  cnpnei  15369  cnptopco  15372  cnptoprest  15389  txcnp  15421  neibl  15641  metcnp3  15661  bj-nntrans  17075