Cannot use 'font' modifier

Hey! Thank you for the library!
I tried to understand function builders following examples but stuck with some strange error when adding `font` modifier.

Here is all the code I have (it is a clean project, Xcode 10.3):
```
NSAttributedString {
"Hello "
.foregroundColor(.black)
// .font(UIFont.systemFont(ofSize: 10.0))
.background(.red)
.underline(.blue)
"World"
.foregroundColor(.green)
.underline(.orange, style: .thick)
}
```
If I comment "font" it compiles and all is good, but if I try ti uncomment I get the error:
Expression type 'NSAttributedString' is ambiguous without more context.