ReactionString:

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   Create a string with a reaction.
   Reactants : 'P' 'H', 'D','T','He3','B11','Li6','He4'
   H and P mean the same.

   Type ReactionString for a demo.
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   Form:
   s = ReactionString( r1, r2 )
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   ------
   Inputs
   ------
   r1        (1,:)    Reactant name
   r2        (1,:)    Reactant name 

   -------
   Outputs
   -------
   s         (1,:)    String

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