Different substitutions in First order logic?

Question

In First order logic the substitution F{x1 / s1}{x2 / s2} ...{xn / sn} can be different from F{x1 / s1, x2 / s2, ... xn / sn}. How these substitutions can be different? Can someone give me example of such substitution? Thanks a lot

Answer 1

(I use the convention that $[\tau/x]$ means "put $\tau$ in place of $x$" or "$\tau$ substituted for $x$.")

Here's a simple example:

$$y[x/y][y/x] = y, \qquad y[x/y,y/x] = x$$