PACK

 PACK (array, mask [,vector])

 Class:  Transformational function - Generic

 Takes elements from an array and packs them into a rank-one array
 under the control of a mask.

 The "mask" must be of logical type and conformable with "array".
 The "vector" must be a rank-one array with the same type and type
 parameters as "array".  Its size must be at least t, where "t" is
 the number of true elements in "mask".  If "mask" is a scalar with
 value true, "vector" must have at least as many elements as there
 are in "array".

 Elements in "vector" are used to fill out the result array if there
 are not enough elements selected by "mask."

 The result is a rank-one array with the same type and type
 parameters as "array".  If "vector" is present, the size of the
 result is that of "vector".  Otherwise, the size of the result is
 the number of true elements in "mask", or the number of elements in
 "array" (if "mask" is a scalar with value true).

 Elements in "array" are processed in array element order to form
 the result array.  Element i of the result is the element of
 "array" that corresponds to the ith true element of "mask".

 If "vector" is present and has more elements than there are true
 values in "mask", any result elements that are empty (because they
 were not true according to "mask") are set to the corresponding
 values in "vector".

 Examples:

 Consider that N is the array |0 8 0|.
                              |0 0 0|
                              |7 0 0|

 PACK (N, MASK=N .NE.  0, VECTOR= (/1, 3, 5, 9, 11, 13/) produces
 the result (7, 8, 5, 9, 11, 13).

 PACK (N, MASK=N .NE.  0) produces the result (7, 8).