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  "body": "\n## Summary\n\nThe arity of an operation, a function or a relation is the number of arguments it takes.\nAn operation of arity $n$ is called $n$-ary: nullary for $0$, unary for $1$, binary for $2$ and ternary for $3$.\nArities are part of every [[signature|WDVVEYD6AT]], and they tell what kind of interpretation each symbol gets in a [[structure|983GZBQYT4]].\n\n## Explanations\n\n### L1 human\n\nThe arity counts the inputs.\nNegation $x \\mapsto -x$ takes one number, so its arity is $1$.\nAddition $(x, y) \\mapsto x + y$ takes two numbers, so its arity is $2$.\nThe relation \"$x$ lies between $y$ and $z$\" compares three points, so its arity is $3$.\nA constant such as $0$ takes no input at all and is seen as an operation of arity $0$.\n",
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