Symbolic substitution (subs) doesn't work with negative expressions

Does the SUBS function have a bug when expressions have a negative sign in them? Why isn't the second output (a - c)?

clear all;
syms a b c

disp( subs(a + 2*b, 2*b, c) );
disp( subs(a - 2*b, 2*b, c) ); % The only difference is the negative sign
disp( subs(a - 2*b, -2*b, c) ); % Works if substituting -2*b instead of 2*b

Output:

a + c  

a - 2*b

a + c

Answer 1

[Azzi Abdelmalek]

Use

disp( subs(a + 2*b, {-2*b,2*b}, {-c c}) )

Answer 2

[Walter Roberson]

You should use MuPAD's op() and type() functions in order to explore the internal structure of each of the expressions. For example it is possible that the expression a - 2*b is stored internally as

_subtract(a, _mult(b, 2))

and that -2*b in the subs() call is being stored internally as

_negate(_mult(b, 2))

or

_mult(b, -2)

As _mult(b, -2) is not a subtree of _subtract(a, _mult(b, 2)) then the subs() would fail.

When I read through the documentation in the symbolic toolbox, I would expect that the representations would be _plus(a, _mult(b, -2)) and _mult(b, -2) in which case the subs() should succeed, but I do not have that toolbox to test with, and the alternate representations I showed are plausible ones that could potentially be triggered by the MATLAB to MuPAD interface.