let V, X, Y be ComplexLinearSpace; :: thesis: ( V is Subspace of X & X is Subspace of Y implies V is Subspace of Y )
assume that
A1: V is Subspace of X and
A2: X is Subspace of Y ; :: thesis: V is Subspace of Y
( the carrier of V c= the carrier of X & the carrier of X c= the carrier of Y ) by A1, A2, Def5;
hence the carrier of V c= the carrier of Y by XBOOLE_1:1; :: according to CLVECT_1:def 5 :: thesis: ( 0. V = 0. Y & the addF of V = the addF of Y || the carrier of V & the Mult of V = the Mult of Y | [:COMPLEX ,the carrier of V:] )
0. V = 0. X by A1, Def5;
hence 0. V = 0. Y by A2, Def5; :: thesis: ( the addF of V = the addF of Y || the carrier of V & the Mult of V = the Mult of Y | [:COMPLEX ,the carrier of V:] )
thus the addF of V = the addF of Y || the carrier of V :: thesis: the Mult of V = the Mult of Y | [:COMPLEX ,the carrier of V:]

proof

set AY = the addF of Y;
set VX = the carrier of X;
set AX = the addF of X;
set VV = the carrier of V;
set AV = the addF of V;
the carrier of V c= the carrier of X by A1, Def5;
then A3: [:the carrier of V,the carrier of V:] c= [:the carrier of X,the carrier of X:] by ZFMISC_1:119;
the addF of V = the addF of X || the carrier of V by A1, Def5;
then the addF of V = (the addF of Y || the carrier of X) || the carrier of V by A2, Def5;
hence the addF of V = the addF of Y || the carrier of V by A3, FUNCT_1:82; :: thesis: verum

end;

set MY = the Mult of Y;
set MX = the Mult of X;
set MV = the Mult of V;
set VX = the carrier of X;
set VV = the carrier of V;
the carrier of V c= the carrier of X by A1, Def5;
then A4: [:COMPLEX ,the carrier of V:] c= [:COMPLEX ,the carrier of X:] by ZFMISC_1:118;
the Mult of V = the Mult of X | [:COMPLEX ,the carrier of V:] by A1, Def5;
then the Mult of V = (the Mult of Y | [:COMPLEX ,the carrier of X:]) | [:COMPLEX ,the carrier of V:] by A2, Def5;
hence the Mult of V = the Mult of Y | [:COMPLEX ,the carrier of V:] by A4, FUNCT_1:82; :: thesis: verum