Concept for the "pseudo-root" of an unrooted tree

[hlapp]

Originally requested by @bendmorrris on CDAO-discuss:

A few of us discussed this briefly at iEvoBio; it would be great to have a concept similar to "has_Root", but for unrooted trees. Some tree formats, such as Newick, represent rooted and unrooted trees identically - an unrooted tree has a "pseudo-root" and is represented the same way as a rooted tree. The position of this pseudo-root is completely arbitrary. If a Newick tree is converted to CDAO and then back to Newick, the resulting tree may appear to be "rooted" differently than the original. While this is technically fine (since it's unrooted, it shouldn't matter where the "pseudo-root" is placed), it may be confusing or irritating to users. Having a property cdao:has_PseudoRoot would ensure that the tree could be retrieved from a triple store and converted back to a Newick tree without changing the position of the pseudo-root - so users would get back a tree that they could easily verify was identical to what they started with.

Currently I'm using has_Root for unrooted trees; this is certainly incorrect, as the pseudo-root is not a root. It's important to distinguish between a true root and what's merely a starting point for convenience but has no real meaning.

See thread on CDAO-discuss for responses.