From Lazarus wiki

Mattias has provided more code for Lazarus project than anyone else, to all areas except maybe some widgetsets. He is the main expert in these areas:

  • Codetools which are used by the IDE to parse pascal sources.
  • Project related code
  • Package related code
  • XML libraries
  • Many LCL classes and functions
  • Many IDE features

... and so on. Search for "Mattias Gärtner" in Lazarus sources.