In my text, many concepts are introduced (defined) by a sentence with 'called'. Examples are: This condition is called eigenvariable condition. This procedure is called the truth table method. A filter defined like that is called a trivial ultrafilter. Should 'called' be followed by an article? I have a feeling that 'called' makes a difference, and that in 'this formula is the interpolant of A and B' the article is fine. Is it so?