\setupinteraction[state=start]

%\useURL [url] [http://www.a.com] [] [x]     % this compiles
%\useURL [url] [http://www.a.com] [] [xy]    % this compiles
%\useURL [url] [http://www.a.com] [] [xyz]    % this compiles
\useURL [url] [http://www.a.com] [] [wxyz]   % this does NOT compile
%\useURL [url] [http://www.a.com] [] [vwxyz]  % this does NOT compile

%\useURL [url] [http://www.ab.com] [] [x]    % this compiles
%\useURL [url] [http://www.ab.com] [] [xy]    % this compiles
%\useURL [url] [http://www.ab.com] [] [xyz]   % this compiles
%\useURL [url] [http://www.ab.com] [] [wxyz]  % this does NOT compile
%\useURL [url] [http://www.ab.com] [] [vwxyz]  % this does NOT compile

%\useURL [url] [http://www.abc.com] [] [x]    % this compiles
%\useURL [url] [http://www.abc.com] [] [xy]   % this compiles
%\useURL [url] [http://www.abc.com] [] [xyz]   % this compiles
%\useURL [url] [http://www.abc.com] [] [wxyz]  % this does NOT compile
%\useURL [url] [http://www.abc.com] [] [vwxyz] % this does NOT compile
 
\starttext
\from[url]
\stoptext