$$T(x) = y $$ $$T(y) = xy$$This transformation has the following successive effect (starting with $x$): $$x \Rightarrow T(x) = y$$ $$ y \Rightarrow T(y) = xy$$ $$ xy \Rightarrow T(x)T(y) = yxy$$ $$ yxy \Rightarrow T(y)T(x)T(y) = xyyxy$$ $$ xyyxy \Rightarrow T(x)T(y)T(y)T(x)T(y) = \dots$$