Allow return x in flatMap (e.g. flatMap { (x: ...) => x })

[greenhat]

From discord https://discord.com/channels/668903786361651200/840313005064585246/902729968331685988 :

I currently have a 

Coll[Coll[(SigmaProp, Coll[Byte])]]

that I am simply trying to flatten into a 

Coll[(SigmaProp, Coll[Byte])]

I tried 

flatMap{ (x : Coll[(SigmaProp, Coll[Byte])]) => x }

 but am running into an error that says: 

Unsupported lambda in flatMap: allowed usage `xs.flatMap(x => x.property)