runge

import des.math.linear.vector;

static struct Point 
{ 
    vec3 pos, vel; 
    mixin( BasicMathOp!"pos vel" );
}

auto v1 = Point( vec3(10,3,1), vec3(5,4,3) );
auto v2 = Point( vec3(10,3,1), vec3(5,4,3) );
assert( v1 + v2 == Point( vec3(20,6,2), vec3(10,8,6) ) );
assert( v1 * 2.0 == Point( vec3(20,6,2), vec3(10,8,6) ) );

Point rpart( in Point p, double time )
{ return Point( p.vel, vec3(0,0,0) ); }

auto v = Point( vec3(10,3,1), vec3(5,4,3) );

double time = 0, ft = 10, step = .01;
foreach( i; 0 .. cast(ulong)(ft/step+1) )
    v = runge( v, &rpart, time+=step, step );

assert( (v.pos - vec3(60,43,31)).len2 < 1e-5 );