Expression error

[Charles]

Hi!
Ihave some question about 'define' expression.
There are 93 ctl files opened in my script,
and my script is:

'reinit'
'/u142/plotting/lib/1985.gs'
'/u142/plotting/lib/1989.gs'
'/u142/plotting/lib/2001.gs'
'set display color white'
'c'
YY=1
sum0=0
while(YY<=93)
if(YY<=26);ys=1985;ye=1986;endif
if(26<YY<=31);ys=85;ye=86;endif
if(31<YY<=54);ys=1989;ye=1990;endif
if(54<YY<=62);ys=89;ye=90;endif
if(62<YY<=93);ys=2001;ye=2002;endif
'set dfile 'YY
  'set t 1'
  'define D'YY'=ave(hgtprs.'YY',time=00Z01DEC'%ys',time=00Z31DEC'%ye')'
  'define SUM'YY'=SUM'YY-1'+D'YY''
YY=YY+1
endwhile

After running it,I got this error:

Data Request Warning: Request beyond file limits
Error from AVE: Error getting grids
Operation Error: Error from ave function
Error occurred at column 1
DEFINE error: Invalid expression

Could anyone tell me what mistake I make?
Thanks for any help!