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This is the way to apply a method to an object.

Very often, instead of f(x), the notation in Python, and therefore in SageMath, will be x.f().

Follow a Python tutorial or a Sage tutorial to learn more.

In the example you mention, sol[0] is an equation, and to extract its right-hand side, you apply the method "rhs" to it, by doing sol[0].rhs().

This is very efficient, especially when you need to apply several operations in a row.

For example, starting from a graph, say you want to compute its adjacency matrix, then get the characteristic polynomial of this matrix, then factor this polynomial.

You can do g.adjacency_matrix().charpoly().factor(), which maybe better shows the sequence of operations, and avoids nested parenthesis, if you compare to writing factor(charpoly(adjacency_matrix(g))).