fuse joins two objects into one by forming a bond. A copy of the object containing the first atom is moved so as to form an approximately reasonable bond with the second, and is then merged with the first object.
fuse (selection1), (selection2)
cmd.fuse( string selection1="(pk1)", string selection2="(pk2)" )
Each selection must include a single atom in each object. The atoms can both be hydrogens, in which case they are eliminated, or they can both be non-hydrogens, in which case a bond is formed between the two atoms.