Theorem ecase23d 1281

Description: Variation of ecased 1280 with three disjuncts instead of two. (Contributed by NM, 22-Apr-1994.) (Revised by Jim Kingdon, 9-Dec-2017.)

Hypotheses

Ref	Expression
ecase23d.1	|- ( ph -> -. ch )
ecase23d.2	|- ( ph -> -. th )
ecase23d.3	|- ( ph -> ( ps \/ ch \/ th ) )

Assertion

Ref	Expression
ecase23d	|- ( ph -> ps )

Proof of Theorem ecase23d

Step	Hyp	Ref	Expression
1		ecase23d.1	. 2 |- ( ph -> -. ch )
2		ecase23d.2	. . 3 |- ( ph -> -. th )
3		ecase23d.3	. . . 4 |- ( ph -> ( ps \/ ch \/ th ) )
4		df-3or 920	. . . 4 |- ( ( ps \/ ch \/ th ) <-> ( ( ps \/ ch ) \/ th ) )
5	3, 4	sylib 120	. . 3 |- ( ph -> ( ( ps \/ ch ) \/ th ) )
6	2, 5	ecased 1280	. 2 |- ( ph -> ( ps \/ ch ) )
7	1, 6	ecased 1280	1 |- ( ph -> ps )

Syntax hints:   -. wn 3   -> wi 4   \/ wo 661   \/ w3o 918

This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-mp 7  ax-ia1 104  ax-ia2 105  ax-ia3 106  ax-in2 577  ax-io 662

This theorem depends on definitions:  df-bi 115  df-3or 920

This theorem is referenced by: (None)