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Suppose you want to write a function which sets the value of the theta array used by all the variants of the q_out function:

func q_out_domain(start, stop, n)
  extern theta;
  theta = span(start, stop, n);

Without the extern statement, theta will be a local variable, whose external value is restored when q_out_domain returns (thus, the function would be a no-op). With the extern statement, q_out_domain has the intended side effect of setting the value of theta. The new theta would be used by subsequent calls to q_out.

Any number of variables may appear in a single extern statement:

extern var1, var2, var3, var4;