private void txtExpressionKeyPressed(java.awt.event.KeyEvent evt) {//GEN-FIRST:event_txtExpressionKeyPressed
        //Pressing enter solves expression.
        if (evt.getKeyCode() == KeyEvent.VK_ENTER && !txtExpression.getText().isEmpty()) {
            //Clear answer and description.
            txtExpression1.setText("");
            lblAnswerDesc.setText("");

            //Calculate
            Solver solver = new Solver(txtExpression.getText(), varData);
            String temp = String.valueOf(solver.solve());
            if (temp.startsWith("ERROR. ")) {
                txtExpression1.setText("ERROR.");
                lblAnswerDesc.setText(temp.substring(7));
            } else {
                txtExpression1.setText(temp);
            }
        }
    }//GEN-LAST:event_txtExpressionKeyPressed