struct PK_INTERVAL_s
    {
    double value[2];
    };
typedef struct PK_INTERVAL_s PK_INTERVAL_t;



This structure represents a real interval.


Specific Errors:
    PK_ERROR_bad_interval           the interval is not a valid subset of
                                    a curve's parameter space


The range represented is that which starts at `value[0]' and goes in the
direction of increasing value to `value[1]'.
`value[0]' must be less than or equal to `value[1]'.
When the two values are equal, the interval has no extent.

When the interval is part of a curve's parameter space, `value[0]' must lie
in the curve's parameter range as given by PK_CURVE_ask_interval.  If the
curve is non-periodic, then `value[1]' must also lie in this range.  If the
curve is periodic, `value[1]' will be in the period above 'value[0]' when the
required interval crosses the seam.