HP 15c - Manual (Page 193)

Section 13: Finding the Roots of an Equation 193
Restriction on the Use of _
The one restriction regarding the use of _ is that _ cannot be
used recursively. That is, you cannot use _ in a subroutine that is
called during the execution of _. If this situation occurs, execution
stops and
Error 7 is displayed. It is possible, however, to use _ with
f thereby using the advanced capabilities of both of these keys.
Memory Requirements
_ requires five registers to operate. (Appendix C explains how they
are automatically allocated from memory.) If five unoccupied registers are
not available, _
will not run and Error 10 will be displayed.
A routine that combines _ and f requires 23 registers of space.
For Further Information
In appendix D, Advanced Use of _, additional techniques and
explanations for using _ are presented. These include:

How _ works.

Accuracy of the root.

Interpreting results.

Finding several roots.

Limiting estimation time.