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Key Structure

Tuesday, September 17, 2013

This section shows conceptually the structure of the FNA keys; the structure of the database used to store those keys is explained later in Node Types & Relationships. In particular, how all the keys at the different ranks fit together to create one key for the whole family.

Targets with a Single Parent - A couplet is a parent-child relationship with two children:
As mentioned in Couplets, Targets and Rows, each couplet choice leads to either another couplet or to a target.

If all targets in a key have a single parent, then there is a unique path between the root of the key (a family, genus, species or subspecies) to each target, usually one rank down from the root.  That is, from each target one can unambiguously traverse back up to the root of the hierarchy.

Targets with Multiple Parents - However, as seen in the Characters Sets and Paths section, some targets in keys can have more than one characters set, which results in multiple parents for those targets. The diagram in that section is part of the Boechera key, which can be redrawn to show B. repanda with two parent rows (35 and 73):

That is, depending on which parent is chosen, a different path and characters set results.

Connection of a Key to a Key at the Next Higher Rank - How keys are connected to the key at the next rank is different for targets with single and multiple parents. An example of each case is shown below.

Silene is a genus target in the Caryophyllaceae key with a single characters set:
The Silene key has the following connections to the Caryophyllaceae key:
The row numbers in this diagram are relative to the key for the rank; in the Key Types and Subkeys section, renumbering of rows in concatenated keys is discussed, but row numbers in the above diagram are before that renumbering. Comparing the Silene key to the Drymaria key using row numbers in the Couplets, Targets and Rows section, row 37 is the root of the Silene key, which is called row 0 in the Drymeria key.

What is important is that the parent for the initial couplet in the Seline key is in the Caryophyllaceae key, so the logic to set up this parent-child relationship is different than if they were in the same key. Row 37 is the Silene target, but it also has a next couplet that is the first couplet in the Silene key. See more on the dual role of nodes in the Node Structure and Types section.

S. scoleri is a target in the Silene key with two characters sets:
By creating a special node that acts as a target for rows 118 and 143 in the Silene key and as a couplet for rows 1 and 2 in the S. scolari key, all other elements in the Silene and S. scolari keys can be treated as normal parent-child relationships.:
 
The Node Structure and Types section discusses how key structures, including these special multiple-characters-set targets, are represented as data structures. In particular, a childList is used for the choices of a couplet and a parentList is used for the parent, or, if there are multiple-characters-sets, for the parents.

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