The different only shows up for field, not with particles. For fields, there are two types of changes that we can make that can keep the Lagrangian unchanged as mentioned at Physics from Symmetry by Jakob Schwichtenberg (2015) chapter "4.5.2 Noether's Theorem for Field Theories - Spacetime":

From defining properties of elementary particles:

From the spacetime theory alone, we can derive the Lagrangian for the free theories for each spin:Then the internal symmetries are what add the interaction part of the Lagrangian, which then completes the Standard Model Lagrangian.

Free to view draft: web.physics.ucsb.edu/~mark/ms-qft-DRAFT.pdf Page presenting it: web.physics.ucsb.edu/~mark/qft.html

Author affiliation: University of California, Santa Barbara.

Number of pages: 616!

Don't redistribute clause, and final version by Cambridge University Press, alas, so corrections will never be merged back: web.physics.ucsb.edu/~mark/qft.html. But at least he's collecing erratas for the published (and therefore draft) versions there.

The book is top-level organized in spin 0, spin half, and spin 1. Quite ominous, really.

The preface states that one of its pedagogical philosophies is to "Illustration of the basic concepts with the simplest examples.", so maybe there is hope after all.

This is a bit "formal hocus pocus first, action later". But withing that category, it is just barely basic enough that 2021 Ciro can understand something.

By: Tobias J. Osborne.

Lecture notes transcribed by a student: github.com/avstjohn/qft

18 1h30 lectures.

Followup course: Advanced quantum field theory lecture by Tobias Osborne (2017).

The mandatory xkcd: xkcd 927: Standards.

Of course, "Ciro Santilli" with quotes, since all of those are either taken directly from others, or had been previously formulated by others.

Some anecdotes.

Ciro Santilli never splits up functions unless there is more than one calling point. If you split early, the chances that the interface will be wrong are huge, and a much larger refactoring follows.

If you just want to separate variables, just use a scope e.g.:

int cross_block_var;

// First step.
{
    int myvar;
}

// Second step.
{
    int myvar;
}

Ciro has seen and had to deal with in his lifetime with two projects that had like 3 to 10 git separate Git repositories, all created and maintained by the same small group of developers of the same organization, even though one could not build without the other. Keeping everything in sync was Hell! Why not just have three directories inside a single repository with a single source of truth?

Another important case: Linux should have at least a C standard library, init system, and shell in-tree, like BSD Operating Systems, as mentioned at: Section "Linux".