Transformation of variable value ranges to constraints

[wangjuwudi]

For example
’‘’
obj: min x1 - x2
s.t. x1 - x2 >= -4
x1 in [0, +∞]
x2 in [0, +∞]
’‘’
we need function transform upper PL to the below PL

'''
obj: min x1 - x2
s.t. x1 - x2 >= -4
x1 >= 0
x2 >= 0
'''