Newbie documentation: Bazaar Merge?

[Ben Finney]

"Stephen J. Turnbull" <turnbull at sk.tsukuba.ac.jp> writes:

> Multiple named heads is an accident waiting to happen in Mercurial,
> and may very well be one in Bazaar as well, depending on the
> implementation and documentation. But that is *because* of the "One
> Explicit Head Per Workspace" philosophy (which Mercurial also
> subscribed to at first). In git colocating branches is […]

Why is “one explicit head per workspace” contrasted with “co-located
branches”? I thought that, even with co-located branches, there's
*still* only one explicit head in the workspace: the head of the current
branch.

Have I misunderstood (again)?