code: public Set<String> getVariableNames()
      {
          final ImmutableSet.Builder<String> sb = ImmutableSet.builder();
          for (final Expression expr : expressions)
              for (final Variable var : expr.getVariables())
                  sb.add(var.getName());
          return sb.build();
      }
docs: Returns an immutable set of variable names contained in the expressions
  of this URI Template.
  
  @return the set of variables names in this template