Assumptions

[schymans]

This solves https://github.com/environmentalscience/essm/issues/56:

from essm.variables import Variable
from sympy import solve
from sympy.physics.units import second
class x(Variable):
     """Positive real variable."""  
     assumptions = {'positive': True, 'real': True}
assert x.is_positive == True    
print(solve(x**2 - 1))

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