Function: variables
Section: conversions
C-Name: variables_vec
Prototype: DG
Help: variables({x}): all variables occuring in object x, sorted by
 decreasing priority. Returns the list of user variables if x is omitted.
Doc:
 returns the list of all variables occuring in object $x$ (all user
 variables known to the interpreter if $x$ is omitted), sorted by
 decreasing priority.
 \bprog
 ? variables([x^2 + y*z + O(t), a+x])
 %1 = [x, y, z, t, a]
 @eprog\noindent The construction
 \bprog
    if (!variables(x),...)
 @eprog\noindent can be used to test whether a variable is attached to $x$.

 If \kbd{varhigher} or \kbd{varlower} are used, it is quite possible to end up
 with different variables (with different priorities) printed in the same
 way: they will then appear multiple times in the output:
 \bprog
 ? y1 = varhigher("y");
 ? y2 = varlower("y");
 ? variables(y*y1*y2)
 %4 = [y, y, y]
 @eprog

Variant:
 Also available is \fun{GEN}{variables_vecsmall}{GEN x} which returns
 the (sorted) variable numbers instead of the attached monomials of degree 1.
