optBS:{{[S;X;rf;rd;t;s] /rd=domestic yield(cc), rf=foreign yield(cc), s=implied volatility
d1:((log S%X)+t0*((s*s*0.5)+rd-rf))%a:s*sqrt t0:t%365f;(S*(exp neg rf*t0)*g d1)-X*(exp neg rd*t0)*g d1-a}

..where g is the gauss function ..
pi:acos -1
g:{abs (neg x>0f)+(1f%sqrt 2f*pi)*(exp -0.5*x*x)*t*0.31938153+t*-0.356563782+t*1.781477937+ ..
.. t*-1.821255978+1.330274429*t:1f%1f+0.2316419*abs x}