let seq1, seq2, seq3 be Real_Sequence; :: thesis: seq1 - (seq2 + seq3) = (seq1 - seq2) - seq3
thus seq1 - (seq2 + seq3) = seq1 + ((- seq2) + ((- 1) (#) seq3)) by Th22
.= (seq1 - seq2) - seq3 by Th13 ; :: thesis: verum