LBL Label (prefix) identifies its suffix (a digit 0, . . . , 9, or top row key, A, . . . , E) as a label in a stored program. A branch to the labeled part of the program can then be done by executing GTO followed by the same suffix. For user-defined functions, the label suffix must be a top row key A, . . . , E). A, B, C, D, E Top Row Keys. These keys can be redefined by inserting a program into memory. When used without a prefix, a top row key finds and executes the corresponding user defined function. Default Functions. When the calculator is turned on, default functions as defined in the window above the top row are automatically inserted into memory. fPRGM clears these functions so that alternate functions may be keyed in. RTN Return.   If executed manually from the keyboard, RTN merely resets the program pointer to the top of memory.   In a stored program, RTN is the logical end of a user defined function.   If a function is executed from the keyboard, RTN stops the program.