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Why is the word “the” used before different styles of calculus?

I've noticed that, when referring to certain branches of calculus, mathematicians sometimes precede the name of those branches with the word "the". For example, "the lambda calculus" or "the predicate calculus".

To be fair, a Google search turns up plenty of references to "lambda calculus" (without "the" as a prefix). But it also turns up an equally large number of references to the phrase with the prefix included. Further, I haven't noticed this with other branches of math. For example, I haven't heard anyone use the phrase "the linear algebra" or "the Euclidean geometry". They just say "linear algebra" or "Euclidean geometry".

My question is, why the difference?