Template:Biochem reaction arrow alt text/doc

This template creates suitable alt text for the Biochem reaction arrow Y XXXX horiz/vert med.svg series of images. It takes parameters in the same order as the images, so the alt text for
 * [[File:Biochem reaction arrow forward YYNN horiz med.svg]]

is

Note that subst and prod can be any values here, so if you get passed a substate on an optional parameter, you can pass   directly to this template, and the arrow will only be mentioned if the parameter is passed and non-blank. The result for this example is