def field(ast, field_name)
      if field_name.nil? || field_name.to_s.empty?
        raise ArgumentError, "Field name cannot be nil or empty"
      end
      ast.xpath("(.//command[ident/@value='#{field_name}']|.//fcall[ident/@value='#{field_name}']/..)")
    end
def field(ast, field_name)
      if field_name.nil? || field_name.to_s.empty?
        raise ArgumentError, "Field name cannot be nil or empty"
      end
      ast.xpath("(.//command[ident/@value='#{field_name}']|.//fcall[ident/@value='#{field_name}']/..)")
    end
Look for a method call with a given name.

 @param ast [Nokogiri::XML::Node] Document to search under
 @param field_name [String] Method name to search for
 @return [Nokogiri::XML::NodeSet]