HP 35s Scientific Calculator - Manual (Page 20)

1-4 Getting Started
Backspacing and Clearing
Among the first things you need to know are how to clear an entry, correct a
number, and clear the entire display to start over.
Keys for Clearing
Key Description
 Backspace.
If an expression is in the process of being entered,
 erases the
character to the left of the entry cursor ( _ ). Otherwise, with a
completed expression or the result of a calculation in line 2,

replaces that result with a zero.  also clears error messages
and exits menus.  behaves similarly when the calculator is in
program-entry and equation-entry modes, as discussed below:
Equation–entry mode:
If an equation is in the process of being entered or edited,
 erases the character immediately to the left of the insert
cursor; otherwise, if the equation has been entered (no insert
cursor present),
 deletes the entire equation.
Program-entry mode:
If a program line is in the process of being entered or
edited,
 erases the character to the left of the insert
cursor; otherwise, if the program line has been entered, 
deletes the entire line.
 Clear or Cancel.
Clears the displayed number to zero or cancels the current
situation (such as a menu, a message, a prompt, a catalog, or
Equation–entry or Program–entry mode).