#1367 matrix_element_add with ratmx : true

[Barton Willis]

Using a user defined value for matrix_element_add and
ratmx bound to true, we get a Lisp error. Example:

(%i1) matrix_element_add : lambda([a,b], max(a,b))$
(%i2) m : matrix([a,b],[c,d])$

This is OK (ratmx : false)

(%i3) m.m;
(%o3) matrix([max(a^2,b*c),max(a*b,b*d)], ...)

This is a bug:

(%i4) m.m, ratmx : true;
Maxima encountered a Lisp error:

[Barton Willis]

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By the time matrix multiplication makes it to multl, it's too late. Maybe we should
do something like

ratmx : true$
matrix_element_mult : lambda([a,b], ...) ---> error