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We only use add, mul and pow operations in the expression tree. Subtraction is represented as addition with an appropriate coefficient and division is multiplication with an appropriate exponent. This makes it easier to handle expressions since you don't need to worry about handling these cases separately.

For example:

sage: var('x,y')
(x, y)
sage: ex = x+y-1
sage: ex.operator()
<built-in function add>
sage: ex.operands()
[x, y, -1]

You can read more about the internal representation of expressions in the relevant part of the GiNaC tutorial.

The patch attached to ticket #13738 wraps some internal GiNaC functions to view the expression tree.