def add_patient(patient)
      raise ArgumentError, "Invalid argument 'patient'. Expected Patient, got #{patient.class}." unless patient.is_a?(Patient)
      # Do not add it again if the patient already belongs to this instance:
      @patients << patient unless @associated_patients[patient.id]
      @associated_patients[patient.id] = patient
    end
def add_patient(patient)
      raise ArgumentError, "Invalid argument 'patient'. Expected Patient, got #{patient.class}." unless patient.is_a?(Patient)
      # Do not add it again if the patient already belongs to this instance:
      @patients << patient unless @associated_patients[patient.id]
      @associated_patients[patient.id] = patient
    end
Adds a Patient to this DataSet.

 @param [Patient] patient a Patient object to be added to this data set