get_cell_cofactors

Get combinational cell pin Shannon expansion cofactors
Usage: @cofactors = get_cell_cofactors($cell, $pin);
$cell: Leaf cell name
$pin: Input pin name
@cofactors: Shannon cofactors for the pin of the cell
            It has two items if the cell is combinational 
            It is empty if the cell is sequential or black-box

Examples:

#1. NAND2X1 A pin 
@ret = get_cell_cofactors("NAND2X1", "A"); # @ret = (1, B);
#2. AOI222X1 A0 pin
@ret = get_cell_cofactors("OAI222X4", "A0"); 
returned @ret = ("!(A1*(B0*(C0+C1)+!B0*(B1*(C0+C1))))", "!(B0*(C0+C1)+!B0*(B1*(C0+C1)))");