code: public static String getEncapsulatingDir(final String aBasePath, final String aPtPath)
              throws InvalidPathException {
          return getEncapsulatingDir(removeBasePath(aBasePath, aPtPath));
      }
docs: Extracts the encapsulating directory from the supplied Pairtree path, using the supplied base path.
  
  @param aBasePath A base path for the Pairtree path
  @param aPtPath The Pairtree path
  @return The name of the encapsulating directory
  @throws InvalidPathException If there is a problem extracting the encapsulating directory