Now the `isEqual` operator does what we call a `deep` comparison,
meaning for every type of `ASTNode` it recursively check if the
structures of the AST tree originating in that node can be considered
equivalent (e.g., two sequence nodes are considered equal if every node
they contain is in turn equal).
This operator gives more space to the short-circuit simplification,
which relies heavily on the `isEqual` operator.