that's not correct. a starts as the string in front of the variable.
for example in "(x+7)^12" it is the empty string "", in "(-x+7)^12" it is the minus sign "-", and in "(0x+7)^12" it is the digit zero "0".
in the second line this string is transformed to the numerical value. so for example "" turns into 1, "-" turn into -1 and "0" turns into 0.
now if a is 0 then the variable isn't relevant and we only have to compute 7^12.