Return a value based on the first traversal that has an element.


coalesce(traversal,traversal\_1, ...)


The coalesce() step evaluates the provided traversals in order and returns the first traversal that emits at least one element.


Create a list of each person and their mean average of stars given for recipe reviews, using coalesce() to either print the mean or print a constant zero value if no reviews are found:

  order().by(select('starCount'), decr)"

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