sorting variables in a sum



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Is there a wxMaxima function to sort the variables of a sum according to the order of another list or set. 
For example sort a sum according to the variables c1,c2,c3,..

(%i1) sum_sort_function(2*c3*x+c1*y+c2+...,[c1,c2,c3,..]);

(%o1) [c1*y+c2+2*c3*x+...]   


Also I think the documentation for the function collectterms should include the term expr to read like:  
Function: collectterms (expr, arg_1, ..., arg_n) 


Ed

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