code: func (b *Builtin) Expr(operands ...*Term) *Expr {
  	ts := make([]*Term, len(operands)+1)
  	ts[0] = NewTerm(b.Ref())
  	for i := range operands {
  		ts[i+1] = operands[i]
  	}
  	return &Expr{
  		Terms: ts,
  	}
  }
docs: // Expr creates a new expression for the built-in with the given operands.