This expression is used to perform baseline based selections. Baseline can be specified as a pair of antenna specifications. Since antenna specification can itself be a list of antennas, the expression allows a rich selection syntax which is simple for simple selections.
ANT in the description below is a comma-separated list of antenna specifications. A baseline specification is an single ANT, ANT followed by an operator or a pair of ANT separated by an operator. A baseline expression is single baseline specification or a semi-colon separated list of baseline specifications.