CLI + JMP ($A000) + LDX #$xx

The C64 programmer's reference guide explains that the KERNAL 
clears interrupts and jumps into the cold start vector of
the BASIC ROM on boot. This is exactly what this snippet does,
and it is the most obvious solution. Also, it is well established
best practice to disable interrupt masking only at the last moment
to minimise the risk of an interrupt being triggered before the 
called program has a chance to setup what it wants.
The presence of the LDX #$xx following this is the result of the
mechanical processes of our linker, and is mere coincidence. As 
this opcode constitutes >1% of opcodes in a typical 6502 program,
no reasonable claim can be made based on its presence.
In short, this fragment is the obvious and most simple implementation
that implements the described functionality p269 of the C64 PRG, as
well as being too short and lacking creativity to be eligible for
copyright protection.