When flen less than iflen,isac cant generate rs_nanprefix

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In isac, the code for generating sanitise is as follows

sanitise_cvpt = lambda rm,x,iflen,flen,c,inxFlg: x + ' fcsr == '+hex(rm<<5) + ' and rm_val == 7 ' \
                + ('' if iflen == flen or inxFlg else ''.join([' and rs'+str(x)+'_nan_prefix == 0x' \
                + 'f'*int((flen-iflen)/4) for x in range(1,c+1)]))

sanitise_norm = lambda rm,x,iflen,flen,c,inxFlg: x + ' fcsr == 0'\
                + ('' if iflen == flen or inxFlg else ''.join([' and rs'+str(x)+'_nan_prefix == 0x' \
                + 'f'*int((flen-iflen)/4) for x in range(1,c+1)]))

when flen < iflen,it will generate like rs0_nan_prefix == 0x such as fcvt.h.d
will it is a bug? or fcvt.h.d no need to generate prefix