defpred S₁[ object ] means $1 is BilinearOperator of X,Y,Z;
consider IT being set such that
A1: for x being object holds
( x in IT iff ( x in Funcs ( the carrier of [:X,Y:], the carrier of Z) & S₁[x] ) ) from XBOOLE_0:sch 1();
take IT ; :: thesis: ( IT is Subset of (RealVectSpace ( the carrier of [:X,Y:],Z)) & ( for x being set holds
( x in IT iff x is BilinearOperator of X,Y,Z ) ) )
for x being object st x in IT holds
x in Funcs ( the carrier of [:X,Y:], the carrier of Z) by A1;
hence IT is Subset of (RealVectSpace ( the carrier of [:X,Y:],Z)) by TARSKI:def 3; :: thesis: for x being set holds
( x in IT iff x is BilinearOperator of X,Y,Z )
let x be set ; :: thesis: ( x in IT iff x is BilinearOperator of X,Y,Z )
thus ( x in IT implies x is BilinearOperator of X,Y,Z ) by A1; :: thesis: ( x is BilinearOperator of X,Y,Z implies x in IT )
assume A2: x is BilinearOperator of X,Y,Z ; :: thesis: x in IT
then x in Funcs ( the carrier of [:X,Y:], the carrier of Z) by FUNCT_2:8;
hence x in IT by A1, A2; :: thesis: verum