code: public void addCodeSystem(CodeSystem theCodeSystem) {
      Validate.notBlank(theCodeSystem.getUrl(), "theCodeSystem.getUrl() must not return a value");
      addToMap(theCodeSystem, myCodeSystems, theCodeSystem.getUrl());
    }
docs: Add a new CodeSystem resource which will be available to the validator. Note that
  {@link CodeSystem#getUrl() the URL field) in this resource must contain a value as this
  value will be used as the logical URL.
  <p>
  Note that if the URL is a canonical FHIR URL (e.g. http://hl7.org/StructureDefinition/Extension),
  it will be stored in three ways:
  <ul>
  <li>Extension</li>
  <li>StructureDefinition/Extension</li>
  <li>http://hl7.org/StructureDefinition/Extension</li>
  </ul>
  </p>