code: private String getCurrentSSpaceFileName() {
          // REMINDER: This instruction is expected to be rare, so rather than
          // save the name and require a lookup every time the current sspace
          // is needed, we use an O(n) call to find the name as necessary
          for (Map.Entry<String,SemanticSpace> e : fileNameToSSpace.entrySet()) {
              if (e.getValue() == current) {
                  return e.getKey();
              }
          }
          return null;
      }
docs: Returns the name of the file form which the current {@code SemanticSpace}
  was loaded, or {@code null} if no semantic space is currently open.
  
  @return the name of the file from which the current space was loaded