__ (two _ characters) or BlankSequence[ ] is a pattern object that can stand for any sequence of one or more Mathematica expressions.
__h or BlankSequence[h] can stand for any sequence of one or more expressions, all of which have head h.
See notes for BlankNullSequence.
See The Mathematica Book: Section 2.3.8.
THIS IS DOCUMENTATION FOR AN OBSOLETE PRODUCT.
SEE THE DOCUMENTATION CENTER
FOR THE LATEST INFORMATION.