$$ Syntax (Echo Expressions) - Spicery/Nutmeg GitHub Wiki

Nutmeg does not support the compound operators such as +=, %= etc. Instead it provides a shorthand for repeating the target of an assignment. Quite simply $$ echoes the left-hand side of an assignment. So instead of:

x += 1

in Nutmeg we write

x <- $$ + 1     ### Syntactic sugar for x <- x + 1

Although it is slightly more verbose than the compound operators, it is still quite brief and has the advantage of working with any function. For example it is perfectly valid to write:

x <- f( $$ ) + 1