Theorem hba1 2151

Description: The setvar x is not free in A. x ph. This corresponds to the axiom (4) of modal logic. Example in Appendix in [Megill] p. 450 (p. 19 of the preprint). Also Lemma 22 of [Monk2] p. 114. (Contributed by NM, 24-Jan-1993.) (Proof shortened by Wolf Lammen, 15-Dec-2017.) (Proof shortened by Wolf Lammen, 12-Oct-2021.)

Assertion

Ref	Expression
hba1	|- ( A. x ph -> A. x A. x ph )

Proof of Theorem hba1

Step	Hyp	Ref	Expression
1		nfa1 2028	. 2 |- F/ x A. x ph
2	1	nf5ri 2065	1 |- ( A. x ph -> A. x A. x ph )

Syntax hints:   -> wi 4   A.wal 1481

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1722  ax-4 1737  ax-5 1839  ax-6 1888  ax-7 1935  ax-10 2019  ax-12 2047

This theorem depends on definitions:  df-bi 197  df-or 385  df-ex 1705  df-nf 1710

This theorem is referenced by:  nfa1OLD  2157  nfaldOLD  2166  nfa1OLDOLD  2207  axi5r  2594  axial  2595  bj-19.41al  32637  bj-modal4e  32705  hbntal  38769  hbimpg  38770  hbimpgVD  39140  hbalgVD  39141  hbexgVD  39142  ax6e2eqVD  39143  e2ebindVD  39148  vk15.4jVD  39150