comes with standard definitions, but also allows to define custom compositions. File $WINHOME/.XCompose:
<Multi_key> <p> : "ä" <Multi_key> <P> : "Ä" <Multi_key> <bracketleft> : "ö" <Multi_key> <braceleft> : "Ö" <Multi_key> <bracketright> : "ü" <Multi_key> <braceright> : "Ü" <Multi_key> <semicolon> : "å" <Multi_key> <colon> : "Å"