Boolean Algebra Simplification Unknown Step Introduce Variables

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I've been trying to interpret a logical proposition for several days. I need to simplify, I can't do it with the laws that I know, but using online tools I can find the result, but in the step by step a method appears that I don't know

The question is: In the next table what does it mean: "introduce Variables" (I know the other laws)

where does (A+A’) come from

the proposition is: [(A’B’) + (A’+C)’]’ (B’C’)’

The Solution is:

[(A’B’)+(A’+C)’]’ (B’C’)’ Defininition
(A+B)(A’+C’’)(B’C’)’ De morgan Law
(AA’ + AC + BA’ + BC) (B’C’)’ Distrib Law
(0 + AC + BA’ + BC) (B’C’)’ Complement Law
(AC + BA’ + BC) (B’C’)’ Identity Law
(AC + BA’ + BC) (B’’+C’’) De morgan Law
ACB + ACC +BA’B + BA’C + BCB + BCC Distrib Law
ACB + CA + BA’B + BA’C + BCB + BCC Idemp Law
AC + BA’B + BA’C + BCB + BCC Absorpt Law
AC + BA’ + BA’C + BCB + BCC Idemp Law
BA’ + AC + BCB + BCC Absorpt Law
BC + BA’ + AC Idemp Law
BC (A+A’) + BA’ + AC Introduce Variables
BCA + BCA’ + BA’ + AC Distrib Law
AC + BA’ Absorpt Law
AC + BA’ Answer

Note: Other sites call this step: Consensus

Thanks a lot, my head is going to explode, I can't find anything on the internet to guide me.