Matlab Simplify function in terms of defined function

I have a long expression I am trying to simplify into known expressions, using symbolic editor.

For example, if I know/define,

M=X/Z
N=Z/Y,

then I would like if I write:

simplify(X/Y) to return M*N.

Is this possible? My expression that I would be simplifying is much longer, but I would like it to return defined "known" functions.

Thanks

Answer 1

[Walter Roberson]

The closest MATLAB gets to that is https://www.mathworks.com/help/symbolic/subexpr.html which is common sub-expression optimization, with no real controls.

The trick I tend to use is

subs(X/Y, {X, Y}, {M*Z, Z/N})

that is, solve the expression M is replacing in terms of one of the other variables and then subs() replacing the other variable with the result. It is a bit clumsy, because it replaces all occurrences of the variable.

children() can help sometimes.