 “Consistently” in this context means that if a term is substituted for one occurrence of a given variable, the same term must be substituted for all occurrences of that variable in both \(s\) and \(t\). We could not for example appeal solely to R5 to justify substituting \(u+v\) for \(x\) in the left hand side of \(x+y = y+x\) and \(v+u\) for \(x\) in the right hand side, though some other rule might permit it.