Chain Hull is a variant of Hull across multiple 2D forms, essentially sequential hulling, then creating a union of the results.

chain_hull( 
    circle(), circle().translate([2,0,0]), ... );   // list of CAG/2D forms
 
let a = [];
a.push(circle()); 
chain_hull( a );                       // array of CAG/2D forms
 
chain_hull({closed: true},             // default is false
   circle(), circle().translate([2, 0, 0]), circle().translate([2, 2, 0]))