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   Computes molecular weight, ratio of specific heats and gas constant for
   some well known fuels. All are for frozen flow.
   If no arguments are entered it will list all possible fuels. You may
   enter a number for fuel or a name, which is not case sensitive.
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   Form:
   [m, g, r, s] = Fuels( fuel )
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   Inputs
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   fuel        (1,:)  Fuel ID or fuel name

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   Outputs
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   m           (1,1)  Molecular weight 
   g           (1,1)  Ratio of specific heats 
   r           (1,1)  Gas constant
   s           (1,1)  Fuel 

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