Unnecessary identifier in VarDecl that starts with "_"

[ovdiiuv]

Differentiating

double fn(double x){
    double __b = x;
    return __b;
}

produces:

double fn_darg0(double x) {
    double _d_x = 1;
    double _d___b = _d_x;
    double __b0 = x;
    return _d___b;
}