docs: def define_symbol(name, open_brace, comma, i, j,
                    close_brace, variables, **kwds):
      r"""Define a nice symbol with matrix indices.
  
      >>> name = "rho"
      >>> from sympy import symbols
      >>> t, x, y, z = symbols("t, x, y, z", positive=True)
      >>> variables = [t, x, y, z]
      >>> open_brace = ""
      >>> comma = ""
      >>> close_brace = ""
      >>> i = 0
      >>> j = 1
      >>> f = define_symbol(name, open_brace, comma, i, j, close_brace,
      ...                   variables, positive=True)
      >>> print f
      rho12(t, x, y, z)
  
      """
      if variables is None:
          return Symbol(name+open_brace+str(i+1)+comma+str(j+1) +
                        close_brace, **kwds)
      else:
          return Function(name+open_brace+str(i+1)+comma+str(j+1) +
                          close_brace, **kwds)(*variables)