def _equation(nodes = (1, 2), dofs = (1, 1), coefficients = (1., 1.), 
              comment = None):
    """
    Returns an Abaqus INP formated string for a given linear equation.
    """
    N = len(nodes)
    if comment == None:
      out = ""
    else:
      out = "**EQUATION: {0}\n".format(comment)  
    out+= "*EQUATION\n  {0}\n  ".format(N)
    out += "\n  ".join([ ",".join([ str(nodes[i]), 
                                  str(int(dofs[i])), 
                                  str(coefficients[i]) ]) for i in range(N)])
    return out
Returns an Abaqus INP formated string for a given linear equation.