A_BETWEEN3()

Calculates the angle formed by 3 pts on the x,y,z plane.          
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A_BETWEEN3() calculates the angle formed by three (3) points in a 
three dimensional plane x,y,z, with coordinates passed as double  
precision numbers:  EXAMPLE:  x1=20.0, x2=40.0, x3=60.0 y1=30.0,  
y2=12.0, y3=70.0 z1=10.0, z2=80.0, z3=33.0                        
angle=a_between(x1,y1,z1,x2,y2,z2,x3,y3,z3)