def<#1>{moseslookforif#1ifmoseslookforif}
defmoseslookforif#1if#2moseslookforif{%
  ifxhfuzz#2hfuzz
    % no if in the argument
    mosescolon#1mosescolon
  else
    mosesifelse#1if#2mosesifelse
  fi
}

defmosescolon#1: #2: #3mosescolon{%
  langle #1:ifxhfuzz#2hfuzzelse#2fi:#3rangle
}
defmosesifelse#1 if #2 else #3ifmosesifelse{%
  langle #1 lhd #2 rhd #3rangle
}

%%% Code possibly to be omitted, if lhd and rhd are already available
fonttenlasy=lasy10
fontsevenlasy=lasy7
fontfivelasy=lasy5
newfamlasyfam
textfontlasyfam=tenlasy
scriptfontlasyfam=sevenlasy
scriptscriptfontlasyfam=fivelasy
mathchardeflhd=numexpr2*"1000+lasyfam*"100+"01relax
mathchardefrhd=numexpr2*"1000+lasyfam*"100+"03relax
%%% end of code to possibly omit

% the example

$<a : b : c>$

$<x if y else z>$

bye