In mathematics, natural numbers are a strict subset of integers, and can be used in any context where an integer is expected. In LIGO, this property does not hold true in general. Instead, a given binary arithmetic operation, say, is defined four times, so it can apply to any combination of natural numbers and integers: this is called overloading, and some programming languages extend it to user-defined functions (e.g. members in C++) -- but not LIGO.
So there are no implicit type casts in LIGO, but we can explicitly
cast natural numbers to integers (this is safe in all contexts where
an integer is valid) by calling the predefined function
inverse cast, from
nat is called in mathematics the
absolute value, or
abs in LIGO.