 The syntactic notion of a single-conclusion consequence relation has been extensively studied by representatives of the Polish school of logic, most notably by Alfred Tarski, who in fact initiated this line of research (see Tarski 1930a,b; cf. also Wójcicki 1988). In view of certain key features of a standard consequence relation it is quite remarkable—as well as important—that any entailment relation \(\vDash_V\) defined as above has the following structural properties (see Ryan and Sadler 1992: 34):