[Bostonian]

I've programmed in C++ only sporadically. Are the functions really declared to have type Error, or am I misreading things? For example one function starts as

   Error
   calibrateEuropean(
     f64    v,     //  option price
     f64    s,     //  stock price

[gituliar]

Good observation!

Basically `Error` is an alias to `std::string`. This way code is more meaningful as returning just std::string is too generic.

It's also my convention to report errors as return arguments in this particular codebase, instead of throwing exceptions for example.