The FIND_FIRST_BIT_CLEAR function locates the  first  bit  in  a
  Boolean array whose value is 0 and returns an integer value that
  specifies the index into the array.

  Syntax:

     FIND_FIRST_BIT_CLEAR( vector [[, start_index]] )

  The 'vector' parameter is a variable of  type  PACKED  ARRAY  OF
  BOOLEAN  with an INTEGER index type.  The optional 'start-index'
  parameter must be an INTEGER expression that indexes the element
  at  the  point at which the search starts.  The 'starting index'
  must be greater than or equal to the vector's lower  bound,  and
  less  than  or  equal  to  1  plus  the  vector's  upper  bound;
  otherwise, a range violation occurs.  If omitted,  the  starting
  index defaults to the vector's first element.

  The FIND_FIRST_BIT_CLEAR function returns a value  indexing  the
  first  element  containing  the  value  0.   If no bit is 0, the
  result is 1 plus the vector's upper bound.  If the vector or the
  indexed  part  of  the  vector  has  a  size of 0, the result is
  start-index.