set B = { c where c is Subset of X : for x, y being Subset of X st [x,y]in F & x c= c holds y c= c } ; A1:
{ c where c is Subset of X : for x, y being Subset of X st [x,y]in F & x c= c holds y c= c } =enclosure_of F
; { c where c is Subset of X : for x, y being Subset of X st [x,y]in F & x c= c holds y c= c } c=bool X