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Complex I

The strange convention of placing negative signs with the complex number I
produces errors with substitutions. For example, the following program

AppendTo[$Echo, "stdout"] 
(* Error in Rules *)
a = I*x/f-I*x/g+z
a = a /.{x_/f-x_/g -> x/h}
(* Error disappears if I is replaced by a variable *)
b = v*x/f-v*x/g+z
b = b /.{x_/f-x_/g -> x/h}

results in

Mathematica (sun3.68881) 1.2 (November 6, 1989) [With pre-loaded data]
by S. Wolfram, D. Grayson, R. Maeder, H. Cejtin,
   S. Omohundro, D. Ballman and J. Keiper
with I. Rivin and D. Withoff
Copyright 1988,1989 Wolfram Research Inc.
 -- Terminal graphics initialized -- 

Out[1]= {stdout}

In[2]:= (* Error in Rules *)

In[3]:= a = I*x/f-I*x/g+z

        I x   -I x
Out[3]= --- + ---- + z
         f     g

In[4]:= a = a /.{x_/f-x_/g -> x/h}

        I x   -I x
Out[4]= --- + ---- + z
         f     g

In[5]:= (* Error disappears if I is replaced by a variable *)

In[6]:= b = v*x/f-v*x/g+z

        v x   v x
Out[6]= --- - --- + z
         f     g

In[7]:= b = b /.{x_/f-x_/g -> x/h}

        v x
Out[7]= --- + z


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