Example 1: Use your own version of TAN:
EXTERNAL TAN T = TAN( 45.0 ) ... END FUNCTION TAN( X ) ... RETURN END
Example 2: Pass a user-defined function name as an argument:
REAL AREA, LOW, HIGH EXTERNAL FCN ... CALL RUNGE ( FCN, LOW, HIGH, AREA ) ... END FUNCTION FCN( X ) ... RETURN END SUBROUTINE RUNGE ( F, X0, X1, A ) ... RETURN END