### SLA_DSEP

Angle Between 2 Points on Sphere

ACTION:
Angle between two points on a sphere (double precision).
CALL:
D = sla_DSEP (A1, B1, A2, B2)
##### GIVEN:
 A1,B1 D spherical coordinates of one point (radians) A2,B2 D spherical coordinates of the other point (radians)

##### RETURNED:
 sla_DSEP D angle between [A1,B1] and [A2,B2] in radians

NOTES:
(1)
The spherical coordinates are right ascension and declination, longitude and latitude, etc., in radians.
(2)
The result is always positive.