/usr/include/freefem++/idp/ is in libfreefem++-dev 3.47+dfsg1-2build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/freefem++/idp/AddLayer2d.idp
/usr/include/freefem++/idp/AddLayer3d.idp
/usr/include/freefem++/idp/DDM-Schwarz-macro.idp
/usr/include/freefem++/idp/DDM-funcs-v2.idp
/usr/include/freefem++/idp/ExtractDofsonBorder.idp
/usr/include/freefem++/idp/Heat3d.idp
/usr/include/freefem++/idp/MPIGMRESmacro.idp
/usr/include/freefem++/idp/MPIplot.idp
/usr/include/freefem++/idp/MeshSurface.idp
/usr/include/freefem++/idp/cube.idp
/usr/include/freefem++/idp/func-max.idp
/usr/include/freefem++/idp/getARGV.idp
/usr/include/freefem++/idp/macro_ddm.idp
/usr/include/freefem++/idp/macro_ddm_substructuring.idp
/usr/include/freefem++/idp/mortar-msh.idp