In CLRS(Cormen Rivest) 3e,while reading Chap-23,page-626,Th-23.1,I can't understand the last part of the proof in page-627."It remains to show....is safe for A." . the line that creates problem is "We have A <= T',since A<= (subset of)T and (x,y)belongs to A. thus ..".We have started with A <= T(some mst) now how come we can say A<= T'. The whole purpose of the proof is absent. Please help me with the proof.