I'm a bit confused by a bash script I'm working with. Here's a simplified bit of the syntax/operation that's confusing me:

```

STACKDIR="/Users/my.name/projects/someproject"
WORKDIR="/Users/my.name/projects/someproject/foo/bar/baz"
SUBPATH="${WORKDIR/$STACKDIR\//}"
echo $STACKDIR
echo $WORKDIR
echo $SUBPATH

```

this outputs

```
/Users/my.name/projects/someproject
/Users/my.name/projects/someproject/foo/bar/baz
foo/bar/baz

```

how does `SUBPATH="${WORKDIR/$STACKDIR\//}"` work to remove `STACKDIR` from the start of `WORKDIR`?