Functions marked with the infix keyword can also be called using the infix notation (omitting the dot and the parentheses for the call). Infix functions must meet the following requirements:
They must be member functions or extension functions.
They must have a single parameter.
The parameter must not accept variable number of arguments and must have no default value.
infix fun Int.add(x: Int): Int { return this + x }
// calling the function using the infix notation
fun main() {
println(1 add 2)
// is the same as
println(1.add(2))
}