input: Write a function to port the given Java code snippet to Python 3.
  
  ### Input:
  public class Foo {
    public int foo( int a, int b ) {
      return a + b;
    }
  }
  
  ### Output:
  def foo(a, b):
   return a + b
  
  assert foo(1, 2)==3
  assert foo(0, 0)==0
  assert foo(-1, 1)==0
  assert foo(5, -3)==2
output: def foo(a, b):
   return a + b