The Impossibility of Double Gallows

[farmerjohn]

Anton, obviously I wasn't clear enough.

I can think of the following sample from Russian. Prefix без [b'ez] is used only before voiced consonants and бес [b'es] only before voiceless ones. Now imagine these are the only prefixes in the language. Obvious conclusion is that these are merely different forms of one prefix.

Now rough sample from English. Endings [t], [id], [d], etc. Each of them is used in certain cases, depending on the previous sound. These cases are mutually exclusive (at least image it's true ). One can conclude that these endings are forms of just one ending (ed).

Return to Voynichese. A glyph, say F, is used in some situations, and never in others. Another glyph, say Y, is used in other situations, but never in some. Additionally, there is no natural restriction which prohibits using F in other situations and it looks as if F could be used in other situations. May be F does appear in other situations, but only in some other form?
So imho the equivalence of F and Y is the first possibility to be checked.