I have recently reconstructed a source code version of Palo Alto ICs BASIC Programmer's Toolkit. This BASIC extender was one of the first firmware accessories for the PET and the first to use the BASIC 'wedge' technique for extending the capabilities of the resident BASIC.
The Toolkit adds the following commands:
AUTO - automatic user specified line numbering
DELETE - delete a range of line from a BASIC program
FIND - locate all program lines which contain a given string or variable
DUMP - list all simple variables and strings with their current values
RENUMBER - renumber the lines of a program with given starting value and step size
APPEND - add a tape program to the end of the current program
TRACE - trace program lines in scroll box on screen
STEP - execute a program one line at a time
OFF - turn off TRACE or STEP mode
HELP - display error line and highlight point of error
The reconstructed source code can be downloaded here. This version is optimized for assembly with the cbA65 assembler described here. An assembled listing may facilitate porting the source to another assembler.