12.6.2.1554
FLNP1
f-l-n-p-one
FLOATING EXT
 
( F: r1 -- r2 ) or ( r1 -- r2 )

r2 is the natural logarithm of the quantity r1 plus one. An ambiguous condition exists if r1 is less than or equal to negative one.

See:
A.12.6.2.1554 FLNP1.

Rationale:
This function allows accurate compilation when its arguments are close to zero, and provides a useful base for the standard logarithmic functions. For example, FLN can be implemented as:

: FLN 1.0E0 F- FLNP1 ;
See:
A.12.6.2.1516 FEXPM1.