Why do most programming languages only have binary equality comparison operators?

Question

In natural languages, we would say \"some color is a primary color if the color is red, blue, or yellow.\"

In every programming language I\'ve seen, that translates

Answer 1

As a mathematician, I would say that the colour is primary if and only if it is a member of the set {red, green, blue} of primary colours.

And this is exactly how you could say in Delphi:

isPrimary := Colour in [clRed, clGreen, clBlue]

In fact, I employ this technique very often. Last time was three days ago. Implementing my own scripting language's interpreter, I wrote

const
  LOOPS = [pntRepeat, pntDoWhile, pntFor];

and then, at a few lines,

if Nodes[x].Type in LOOPS then

The Philosophical Part of the Question

@supercat, etc. ("As to why nobody's done that, I don't know."):

Probably because the designers of programming languages are mathematicians (or, at least, mathematically inclined). If a mathematician needs to state the equality of two objects, she would say

X = Y,

naturally. But if X can be one of a number of things A, B, C, ..., then she would define a set S = {A, B, C, ...} of these things and write

X ∈ S.

Indeed, it is extremely common that you (mathematicians) write X ∈ S, where S is the set

S = {x ∈ D; P(x)}

of objects in some universe D that has the property P, instead of writing P(X). For instance, instead of saying "x is a positive real number", or "PositiveReal(x)", one would say x ∈ ℝ⁺.