Function embed 

pub fn embed(
    indexed: &IndexedDocument<'_>,
    embed: Embed<'_>,
    fresh: &mut Allocator,
    builder: &mut CommitBuilder,
) -> BlockId

Inventory row “Embed”: a document becomes a block of this one.

Returns the block, which is what the caller selects — the graft inside it is left unnamed, as a paste leaves the structure that travelled inside its roots.