code: public void addVarRefExpr(final INodeReadTrx mTransaction, final String mVarName) {
  
          final VariableAxis axis = (VariableAxis)mVarRefMap.get(mVarName);
          if (axis != null) {
              getExpression().add(new VarRefExpr(mTransaction, axis));
          } else {
              throw new IllegalStateException("Variable " + mVarName + " unkown.");
          }
  
      }
docs: Adds a VarRefExpr to the pipeline. This Expression holds a reference to
  the current context item of the specified variable.
  
  @param mTransaction
  the transaction to operate on.
  @param mVarName
  the name of the variable