Change naming (#3678)

* Change naming

* x to a

src/prelude/relation.ts

@@ -1,5 +1,5 @@
-export type Predicate<T> = (x: T) => boolean;
+export type Predicate<T> = (a: T) => boolean;
 
-export type Relation<T, U> = (x: T, y: U) => boolean;
+export type Relation<T, U> = (a: T, b: U) => boolean;
 
 export type EndoRelation<T> = Relation<T, T>;