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Why shouldn't I start variable names with underscores?

Identifier names beginning with two underscores or an underscore followed by a capital letter are reserved
for use by the compiler or standard library functions wherever they appear. In addition, all identifier names
beginning with an underscore followed by anything are reserved when they appear in file scope (when they
are not local to a function).

If you use a reserved identifier for a variable name, the results are undefined (your program might not
compile, or it might compile but crash). Even if you are lucky enough to pick an identifier that is not currently
used by your compiler or library, remember that these identifiers are reserved for possible use later. Thus,
it's best to avoid using an underscore at the beginning of variable and function names.