TYI atom not supported

[einstein95]

Spellbreaker's Z6.ZIL has this commented-out code

<REPEAT (CHR)
        <PRINC "Debugging? (Y or N): " ,OUTCHAN>
        <SET CHR <TYI>>
        <COND (<MEMQ .CHR "Yy ">
               <PRINC " Debugging!
" ,OUTCHAN>
               <RETURN <SETG ZDEBUGGING? T>>)
              (<MEMQ .CHR "Nn">
               <PRINC " No debugging!
" ,OUTCHAN>
               <RETURN <SETG ZDEBUGGING? <>>>)
              (ELSE
               <PRINC " ??
" ,OUTCHAN>)>>

Attempting to compile with this code uncommented causes an error with

Debugging? (Y or N): [error MDL0200] z6.zil:19: calling unassigned atom: TYI
  in SET called at z6.zil:19
  in REPEAT called at z6.zil:17

This is an atom from MDL, from the manual:

<TYI 〈terminal-input:channel〉>

returns one CHARACTER from 〈channel〉 (optional, .INCHAN by default) when it is typed, rather than after $ (ESC) is typed, as is the case with READCHR. The following example echos input characters as their ASCII values, until a carriage-return is typed:

<REPEAT (( FOO <TTYECHO . INCHAN <>>))
    <AND <==? 13 <PRINC <ASCII <TYI .INCHAN >>>>
        <RETURN <TTYECHO . INCHAN T>>>>