Add "choose" syntax

[cl4cnam]

I intend to add a syntaxic sugar for:

parallel(select N) ||
||==============
	myFunction(arg1,...,argM)
...--------
	restOfBranch1
||=============
	myFunction(arg1b,...,argMb)
...-------
	restOfBranch2

that would be:

choose N by myFunction
	* arg1,...,argM
		restOfBranch1
	* arg1b,...,argMb
		restOfBranch2