func (p *Plus) Interpret(variables map[string]Expression) int {
	return p.leftOperand.Interpret(variables) + p.rightOperand.Interpret(variables)
}
Interpret interprets by adding the left and right variables.
package behavioral
import (
	"strings"
)
type Expression interface {
	Interpret(variables map[string]Expression) int
}
type Integer struct {
	integer int
}
func (n *Integer) Interpret(variables map[string]Expression) int {
	return n.integer
}
type Plus struct {
	leftOperand  Expression
	rightOperand Expression
}