1:: | |- (. A e. V ->. A e. V ).
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2:1: | |- (. A e. V ->. [_ A / x ]_ V = V ).
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3:2: | |- (. A e. V ->. ( [_ A / x ]_ C X. [_ A /
x ]_ V ) = ( [ A / x ]_ C X.V ) ).
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4:1: | |- (. A e. V ->. [ A / x ]_ ( C X.V ) =
( [ A / x ]_ C X. [_ A / x ]_ V ) ).
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5:3,4: | |- (. A e. V ->. [ A / x ]_ ( C X.V ) =
( [ A / x ]_ C X.V ) ).
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6:5: | |- (. A e. V ->. ( [ A / x ]_ B i^i [_ A /
x ]_ ( C X.V ) ) =
( [ A / x ]_ B i^i ( [_ A / x ]_ C X.V ) ) ).
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7:1: | |- (. A e. V ->. [ A / x ]_ ( B i^i ( C X.
_V ) ) = ( [_ A / x ]_ B i^i [_ A / x ]_ ( C X.V ) ) ).
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8:6,7: | |- (. A e. V ->. [ A / x ]_ ( B i^i ( C X.
_V ) ) = ( [_ A / x ]_ B i^i ( [_ A / x ]_ C X.V ) ) ).
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9:: | ` |- ( B |`C ) = ( B i^i ( C X. V ) )
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10:9: | ` |- A. x ( B |`C ) = ( B i^i ( C X.V ) )
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11:1,10: | ` |- (. A e. V ->. [ A / x ]_ ( B |`C ) =
[_ A / x ]_ ( B i^i ( C X.V ) ) ).
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12:8,11: | ` |- (. A e. V ->. [ A / x ]_ ( B |`C )
= (
[_ A / x ]_ B i^i ( [_ A / x ]_ C X.V ) ) ).
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13:: | ` |- ( [ A / x ]_ B |`[_ A / x ]_ C ) = (
[_ A / x ]_ B i^i ( [_ A / x ]_ C X.V ) )
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14:12,13: | ` |- (. A e. V ->. [ A / x ]_ ( B |`C ) =
(
` [_ A / x ]_ B |`[_ A / x ]_ C ) ).
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qed:14: | ` |- ( A e. V -> [_ A / x ]_ ( B |`C ) = (
` [_ A / x ]_ B |`[_ A / x ]_ C ) )
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