Change the order of variables : reorder

reorder takes two arguments : an expression and a vector of variable names.
reorder expands the expression according to the order of variables given as second argument.
Input :

reorder(x^2+2*x*a+a^2+z^2-x*z,[a,x,z])

Output :

a^2+2*a*x+x^2-x*z+z^2

Warning :
The variables must be symbolic (if not, purge them before calling reorder)