Hello fellow ConTeXies, 

since April 2019 it is possible to use \setupregister with
compress=text which is a fine addition and has its use cases.
Unfortunately, I am confronted with the demand to style the registers
with 'f.' for a following single page and else with page ranges. Since
the Definition of \registerpagerange is not well documented I don't
know which parameter to check for the conditional statement when
rewriting it. Could you please give me a hint? 

In ConTeXt that shipped with TeXlive 2015 it worked that way:
\unexpanded\def\registerpagerange#1#2#3#4#5#6#7%
    {\registerpageseparator
     \global\setconstant\c_strc_registers_page_state\plusone
     \dostarttagged\t!registerpagerange\empty
     \dostarttagged\t!registerfrompage\empty
     \withregisterpagecommand{#1}{#2}{#3}{#4}%
     \dostoptagged
     \ifnum\the\numexpr#6-#3\relax=1
       f.%
     \else
       \registeronepagerangeseparator
       \dostarttagged\t!registertopage\empty
       \withregisterpagecommand{#1}{#5}{#6}{#7}%
       \dostoptagged
     \fi
     \dostoptagged}

With best regards
Stefan