 
   July 1, 1994                                                        SIS(1)
 
   read_eqn [-a] [filename]
 
     Read a set of logic equations in the format expected by eqntott(1).
     Each equation becomes a node in the logic network.
 
     INORDER and OUTORDER can be used to specify the primary inputs and pri-
     mary outputs for the network.  If neither is given, then primary inputs
     are inferred from signals which are not driven, and primary outputs are
     inferred from signals which do not have any fanout.
 
     The equations are of the form "<signal> = <expr> ;".  For reference, the
     equation format uses the operators:
 
                           ___________________________
                          | ()          grouping     |
                          | != (or ^)   exclusive-or |
                          | ==          exclusive-nor|
                          | !           complement   |
                          | & (or *)    boolean-and  |
                          ||_|__(_o_r__+_)_____b_o_o_l_e_a_n_-_o_r_____||
     As a simple extension to eqntott, juxtaposition of two operands stands
     for boolean-and, and ' used as a post-fix operator stands for comple-
     ment.  Hence,
               F = a*!b + c*!d ;
     and
               F = a b' + c d' ;
     represent the same equation.
 
     Note that eqntott and read_eqn treat the intermediate nodes of a network
     slightly differently.  read_eqn will not make an intermediate node a
     primary output unless it also appears in the OUTORDER list.  Also, the
     resulting network is a multiple-level network with all of the intermedi-
     ate signals preserved.  Finally, eqntott is order-dependent in that it
     requires signals to be defined before they can be used again; read_eqn
     relaxes this condition.
 
     The -a option specifies that the new network should be appended to the
     current network.  Functions are associated between the two networks
     using the long names of each network.  Name conflicts (where two func-
     tions attempt to define the same name) generate warning messages and are
     resolved by renaming the signal from the new network.
 
     The -s option, though accepted, has no effect on read_eqn and is instead
     reserved for the read_pla command.  Note that since the characters '('
     and ')' are used for grouping, they cannot be part of a signal name.
 
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