text: from pysb import Model, Monomer, Parameter, Compartment, Rule, Initial, Observable
  
  Model()
  
  Compartment('compartment')
  
  Monomer('A_')
  Monomer('B_')
  
  Parameter('a0', 1.0)
  Parameter('b0', 1.0)
  Parameter('k1', 0.0)
  Parameter('k2', 0.0)
  
  Rule('conversion', A_() ** compartment | B_() ** compartment, k1, k2)
  
  Initial(A_() ** compartment, a0)
  Initial(B_() ** compartment, b0)
  
  Observable("A", A_())
  Observable("B", B_())
  