code: @Override
      public Object visitConcatenation(ExcellentParser.ConcatenationContext ctx) {
          String arg1 = Conversions.toString(visit(ctx.expression(0)), m_evalContext);
          String arg2 = Conversions.toString(visit(ctx.expression(1)), m_evalContext);
          return arg1 + arg2;
      }
docs: expression: expression AMPERSAND expression