<h3>union function</h3>
Returns a geometry that represents the point set union of the geometries.

<h4>Syntax</h4>
<pre>union( a, b )</pre>

<h4>Arguments</h4>
a &rarr; geometry
b &rarr; geometry

<h4>Example</h4>
<pre> geomToWKT( union( geomFromWKT( 'POINT(4 4)' ) , geomFromWKT( 'POINT(5 5)' ))  )   &rarr; returns MULTIPOINT(4 4, 5 5)</pre>
