Below is a minimal not-working example extracted from 
"extensions-001.tex" in the fonts section of the test suite. From what I 
understand of the code, the second line should read "XPrXcXdXPrX" 
instead of "abracadabra". But it doesn't, it is still "abracadabra". 
Compiling the original extensions-001 gives me seven lines of 
"abracadabra". What am I doing wrong? I'm running ConTeXt version 
2015.05.18 12:26 MKIV current fmt: 2016.4.28 from the debian/ubuntu 
texlive packets.

\startluacode

fonts.handlers.otf.addfeature {

name = "stest",

type = "substitution",

data = {

a = "X",

b = "P",

}

}

\stopluacode

\definefontfeature[stest][stest=yes]

\starttext

\startTEXpage

\definedfont[file:DejaVuSerif.ttf*default]%

abracadabra\par

{\addff{stest}abracadabra\par}

\stopTEXpage

\stoptext