Changing the order of variables: reorder

The reorder command rewrites an expression, based on the priority of variables.

reorder takes two arguments:
- expr, an expression.
- vars, a vector of variable names.
reorder(expr,vars) expands expr according to the order of variables given in vars.

Example.
Input:

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

Output:

a²+2 a x+x²−x z+z²

Warning.
The variables must be symbolic (if not, purge them (see Section 5.4.8) before calling reorder.