I am reading "Boolean Algebras with Operators part II" by Bjarni Jonsson and Alfred Tarski. On theorem 4.10 (p.132-133), they refer to a relation algebra $\mathfrak{A}$ being "simple" and proves that it is equivalent to $\mathfrak{A}$ having no ideal elements different from 0 and 1. What definition of "simple" is being used in this context? The article is in JSTOR, by the way.
What does it mean for a relation algebra to be simple?
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