Linking subtypes are explicitly restricted by smiles and linker_labels in chem templates

[rwxayheee]

Because the linking subtypes are explicitly restricted by smiles (atoms with implicit hydrogens) and the linker_labels. A new subtype needs to be created for a different type of linking.

To have a distinguishable subtype name, the 5' to 3' notation needs to include:
p <-- phosphate, if any
5' <-- intramol linker position to phosphate
DA <-- type of base name
3' <-- linker position to next nucleotide

p5'DA3' will be the standard embedded form of Adenosine in DNA. A short sequence may consist of:

(5'DT3') - (p5'DA3') - (p5'DC3') - (p5'DG3')

Currently they're named:

(DT5) - (DA) - (DC) - (DG3)