1:: | |- (. A e. C ->. A e. C ).
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2:1: | |- (. A e. C ->. [_ A / x ]_ { y } = {
y } ).
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3:1: | |- (. A e. C ->. [_ A / x ]_ ( F " { B
} ) = ( [_ A / x ]_ F " [_ A / x ]_ { B } ) ).
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4:1: | |- (. A e. C ->. [_ A / x ]_ { B } = {
[_ A / x ]_ B } ).
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5:4: | |- (. A e. C ->. ( [_ A / x ]_ F " [_ A
/ x ]_ { B } ) = ( [_ A / x ]_ F " { [_ A / x ]_ B } ) ).
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6:3,5: | |- (. A e. C ->. [_ A / x ]_ ( F " { B
} ) = ( [_ A / x ]_ F " { [_ A / x ]_ B } ) ).
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7:1: | |- (. A e. C ->. ( [. A / x ]. ( F " {
B } ) = { y } <-> [_ A / x ]_ ( F " { B } ) = [_ A / x ]_ { y } ) ).
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8:6,2: | |- (. A e. C ->. ( [_ A / x ]_ ( F " {
B } ) = [_ A / x ]_ { y } <-> ( [_ A / x ]_ F " { [_ A / x ]_ B } )
= { y } ) ).
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9:7,8: | |- (. A e. C ->. ( [. A / x ]. ( F " {
B } ) = { y } <-> ( [_ A / x ]_ F " { [_ A / x ]_ B } ) = { y } )
).
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10:9: | |- (. A e. C ->. A. y ( [. A / x ]. ( F
" { B } ) = { y } <-> ( [_ A / x ]_ F " { [_ A / x ]_ B } ) =
{ y } ) ).
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11:10: | |- (. A e. C ->. { y | [. A / x ]. ( F
" { B } ) = { y } } = { y | ( [_ A / x ]_ F " { [_ A / x ]_ B } ) =
{ y } } ).
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12:1: | |- (. A e. C ->. [_ A / x ]_ { y | ( F
" { B } ) = { y } } = { y | [. A / x ]. ( F " { B } ) = { y } } ).
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13:11,12: | |- (. A e. C ->. [_ A / x ]_ { y | ( F
" { B } ) = { y } } = { y | ( [_ A / x ]_ F " { [_ A / x ]_ B } ) =
{ y
} } ).
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14:13: | |- (. A e. C ->. U. [_ A / x ]_ { y | (
F " { B } ) = { y } } = U. { y | ( [_ A / x ]_ F "
{ [_ A / x ]_ B } ) =
{ y } } ).
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15:1: | |- (. A e. C ->. [_ A / x ]_ U. { y | (
F " { B } ) = { y } } = U. [_ A / x ]_ { y | ( F " { B } ) =
{ y } } ).
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16:14,15: | |- (. A e. C ->. [_ A / x ]_ U. { y | (
F " { B } ) = { y } } =
U. { y | ( [_ A / x ]_ F " { [_ A / x ]_ B } ) =
{ y } } ).
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17:: | |- ( FB ) =
U. { y | ( F " { B } ) =
{ y } }
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18:17: | |- A. x ( FB ) = U. { y | ( F " { B
} ) = { y } }
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19:1,18: | |- (. A e. C ->. [_ A / x ]_ ( FB )
= [_ A / x ]_ U. { y | ( F " { B } ) = { y } } ).
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20:16,19: | |- (. A e. C ->. [_ A / x ]_ ( FB )
= U. { y | ( [_ A / x ]_ F " { [_ A / x ]_ B } ) = { y } } ).
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21:: | |- ( [_ A / x ]_ F[_ A / x ]_ B ) =
U. { y | ( [_ A / x ]_ F " { [_ A / x ]_ B } ) = { y } }
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22:20,21: | |- (. A e. C ->. [_ A / x ]_ ( FB )
= ( [_ A / x ]_ F[_ A / x ]_ B ) ).
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qed:22: | |- ( A e. C -> [_ A / x ]_ ( FB ) =
( [_ A / x ]_ F[_ A / x ]_ B ) )
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