export function isFunction(x: any): x is Function
export function isGenerator(x: any): boolean

export function toFunction(x): Function

export function isFunctionWithKeys(x: Function): true
export function isFunctionWithKeys(x: any): boolean

// not undefined, null
export function isSafe(
  x: any
): x is string | Function | boolean | Object | Array<any>

// noop
export function NO_OP(): void