Selection expressions, also known as “sexes” in recutils jargon, are infix expressions that can be applied to a record. A “sex” is a predicate which selects a subset of records within a recfile. They can be simple expressions involving just one operator and a pair of operands, or complex compound expressions with parenthetical sub-expressions and many operators and operands. One of their most common uses is to examine records matching a particular set of conditions.