This is from a talk by Edsger W. Dijkstra, "How Computing Science created a new mathematical style", 4 March 1990:
Almost all formalisms used daily by the classical mathematician are at least ambiguous. But that does not hurt the classical mathematician because he does nothing with a formula without interpreting it and part of his professional competence consists in subconsciously rejecting all unintended interpretations. (If you show him
sin($\alpha+\beta$) versus sin(s+i+n)
it depends on his sense of humour whether he is amused.) The manipulation of uninterpreted formulae requires unambiguous formalisms [...]
Tragically, I am not quite sure what is so funny. Concatenating three English letters together with plus signs to make a function name is amusing I suppose, but I get the feeling there is more going on here, and I am not able to put it into words.
Is anyone able to explain these expressions, and why they could be considered humorous?