Start with the narrowest useful interface

[hadley]

• Ensures you get good error messages if unexpected input is supplied.

• Makes the interface maximally extensible, because it's easy to add new functionality; it's hard to take away (or modify) existing functionality.

• Enables you to make a more considered decision about what is important.

[hadley]

Need to carefully balance user friendliness for good inputs with informative errors for bad inputs. The more flexible you make the input specification, the greater the chance that a bad input will silently return an incorrect value.

[hadley]

Related to #141