USR() SCRN?

[lyricalnanoha@dosius.ath.cx (Steve Nickolas)]

The methodology for handling USR applies the same to EHBASIC (where I intend
to use it) and FPBASIC (which is the dominant BASIC interpreter around these
parts), so perhaps someone knows.  I want to implement SCRN( as a USR
function (so as not to have to muddle with the token table again), but every
way I am attempting to implement it is causing me a Syntax Error.

The code in FPBASIC is:

DEF9: 20 B1 00      screen:   jsr       chrget
DEFC: 20 EC F1                jsr       plotfns   ; get column and row
DEFF: 8A                      txa                 ; row
DF00: A4 F0                   ldy       first     ; column
DF02: 20 71 F8                jsr       scrn      ; get 4-bit color there
DF05: A8                      tay
DF06: 20 01 E3                jsr       sngflt    ; convert Y to real in FAC
DF09: 4C B8 DE                jmp       chkcls    ; require ')'

-uso.