Function offset_shell_rounded 

pub fn offset_shell_rounded(
    source: &BrepSolid,
    opening_face_ids: &[u64],
    distance: f64,
) -> Result<BrepSolid, KernelRefusal>

source shelled to a wall distance thick, joined by arcs, open at the flat faces opening_face_ids. The sign is crate::offset_shell’s: a positive distance puts the wall inside the solid, a negative one outside.

Inward, the wall is the solid less every point of it at least the distance from its kept faces. Outward, it is every point outside the solid within the distance of its kept faces, ended by the openings’ planes. With no opening the result is the solid’s own shell and the parallel one, inside or outside it.

Refuses an opening that is not flat, an inward wall with two openings that share an edge, and whatever the parallel offset or the Booleans refuse.