In Kotlin, an infix method can be called without the `.` operator, and without the parens surrounding arguments.

    100 money "USD"

is the same as

    100.money("USD")

`Number.money(currency: String): Money` is an extension method; Kotlin provides syntactic sugar which allows you to "add" methods to existing classes. The above is basically equivalent to:

    fun money(count: Number, currency: String): Money