code: @Nullable
    public Expr get(final String functionName, final List<Expr> args)
    {
      final ExprMacro exprMacro = macroMap.get(StringUtils.toLowerCase(functionName));
      if (exprMacro == null) {
        return null;
      }
  
      return exprMacro.apply(args);
    }
docs: Returns an expr corresponding to a function call if this table has an entry for {@code functionName}.
  Otherwise, returns null.
  
  @param functionName function name
  @param args         function arguments
  
  @return expr for this function call, or null