HP 33s - Manual (Page 221)

Solving and Integrating Programs 14–11
Restrictions on Solving and Integrating
The SOLVE variable and ³ FN d variable instructions cannot call a routine that
contains another SOLVE or ³ FN instruction. That is, neither of these instructions
can be used recursively. For example, attempting to calculate a multiple integral
will result in an
³ 1 ³ 2 error. Also, SOLVE and ³ FN cannot call a routine that
contains an /label instruction; if attempted, a # !# or ³ 
!# error will be returned. SOLVE cannot call a routine that contains an
³ FN instruction (produces a #1 ³ 2 error), just as ³ FN cannot call a
routine that contains a SOLVE instruction (produces an ³ 1 #2 error).
The SOLVE variable and ³ FN d variable instructions in a program use one of the
seven pending subroutine returns in the calculator. (Refer to "Nested Subroutines"
in chapter 13.)
The SOLVE and ³ FN operations automatically set Decimal display format.