node_modules/rambda/src/union.js

import { cloneList } from './_internals/cloneList.js'
import { includes } from './includes.js'

export function union(x, y){
  if (arguments.length === 1) return _y => union(x, _y)

  const toReturn = cloneList(x)

  y.forEach(yInstance => {
    if (!includes(yInstance, x)) toReturn.push(yInstance)
  })

  return toReturn
}