code: public void addVariableExpr(final INodeReadTrx mTransaction, final String mVarName) {
  
          assert getPipeStack().size() >= 1;
  
          final AbsAxis bindingSeq = getPipeStack().pop().getExpr();
  
          final AbsAxis axis = new VariableAxis(mTransaction, bindingSeq);
          mVarRefMap.put(mVarName, axis);
  
          if (getPipeStack().empty() || getExpression().getSize() != 0) {
              addExpressionSingle();
          }
          getExpression().add(axis);
      }
docs: Adds a variable expression to the pipeline. Adds the expression that will
  evaluate the results the variable holds.
  
  @param mTransaction
  Transaction to operate with.
  @param mVarName
  name of the variable