func (variable *Variable) Unset() {
	variable.Empty = true
	variable.Value = nil
}
Unset the variable
package centipede
import "fmt"
type VariableName string
type VariableNames []VariableName
func (varnames *VariableNames) Contains(varname VariableName) bool {
	for _, item := range *varnames {
		if item == varname {
			return true
		}
	}
	return false
}
type Variable struct {
	Name   VariableName
	Value  interface{}
	Domain Domain
	Empty  bool
}
func NewVariable(name VariableName, domain Domain) Variable {
	return Variable{name, 0, domain, true}
}
func (variable *Variable) SetValue(value interface{}) {
	variable.Value = value
	variable.Empty = false
}