code: func (c *Call) After(preReq *Call) *Call {
  	c.t.Helper()
  
  	if c == preReq {
  		c.t.Fatalf("A call isn't allowed to be its own prerequisite")
  	}
  	if preReq.isPreReq(c) {
  		c.t.Fatalf("Loop in call order: %v is a prerequisite to %v (possibly indirectly).", c, preReq)
  	}
  
  	c.preReqs = append(c.preReqs, preReq)
  	return c
  }
docs: // After declares that the call may only match after preReq has been exhausted.