Daniel Gröber (dxld) authored Jul 08, 2019 and Marge Bot committed Feb 17, 2021

The new 'sep' field links a stackElement to it's "parent". That is the
stackElement containing it's parent closure.

Currently not all closure types create long lived elements on the stack so
this does not cover all parents along the path to the root but that is
about to change in a future commit.