The interval operator has a lower precedence than, for example, binary
operations. This was unexpected and incorrectly implemented in the
output functions. For now, this is fixed by enclosing intervals in
parentheses to avoid misinterpretations.
The existing unit tests are adjusted to the updated output format.
The equality check is used within a simplification rule that turns
biconditionals into simple implications in special cases. This adds some
unit tests that cover this simplification rule as well as the equality
check implementation.
