6.1.2210
SIGN
 
CORE
 
( n -- )

If n is negative, add a minus sign to the beginning of the pictured numeric output string. An ambiguous condition exists if SIGN executes outside of a <# #> delimited number conversion.

Testing:
: GP2 <# -1 SIGN 0 SIGN -1 SIGN 0 0 #> S" --" S= ;
T{ GP2 -> <TRUE> }T