The grammar is read from standard input, and the text is written to standard output. Two runs using the same seed will produce the same output. If this argument is not given, a different seed is used on every run. The random-number generator seed is written to stderr before generation begins. The first production is assumed to define the start symbol of the grammar.
Identifiers are nonterminal symbols, string constants are terminals.
The real-constant is the probability of this RHS being chosen when the production's LHS is being expanded. The difference between 1 and the sum of the probabilities for all RHS's which have probabilities specified is divided among the RHS's without probabilities specified.