docs: function toFn (value) {
    if (typeof value === 'function') return value
  
    var str = Array.isArray(value) ? value : String(value)
    var obj = Object.create(null)
  
    for (var i = 0; i < str.length; i++) {
      obj[str[i]] = true
    }
  
    return function (char) {
      return obj[char]
    }
  }