HP 35s Scientific Calculator - Manual (Page 217)

Programming Techniques 14-7
Comparison tests. These compare the X–and Y–registers, or the X–register
and zero.
Flag tests. These check the status of flags, which can be either set or clear.
Loop counters. These are usually used to loop a specified number of times.
Tests of Comparison (x?y, x?0)
There are 12 comparisons available for programming. Pressing   or 
 displays a menu for one of the two categories of tests:
x?y for tests comparing x and y.
x?0 for tests comparing x and 0.
Remember that x refers to the number in the X–register, and y refers to the number in
the Y–register. These do not compare the variables X and Y. You can use x?y and
x?0 to compare two numbers, if one of these isn't real number, it will return an error
message  .
Select the category of comparison, then press the menu key for the conditional
instruction you want.
The Test Menus
If you execute a conditional test from the keyboard, the calculator will display 
or .
For example, if x =2 and y =7, test x <y .
x?y x?0
≠ for x ≠ y? ≠ for x≠0?
≤ for x≤y? ≤ for x≤0?
< for x<y? < for x<0?
> for x>y? > for x>0?
≥ for x≥y? ≥ for x ≥0?
 for x=y?  for x=0?