Model is unsuitably constrained (i.e. there were still some variables left to be computed, and some unusable equations left over).
At the time of the error, the following variables were found:
 * Undefined: i_K in component potassium_channel
 * Undefined: g_K1 in component potassium_channel
 * Undefined: g_K2 in component potassium_channel
 * Undefined: g_K_add in component potassium_channel
 * Undefined: time in component potassium_channel
 * Undefined: V in component potassium_channel
 * Undefined: n in component potassium_channel
The following equations couldn't be used:
 * Equation in apply element with id i_K_calculation.
 * Equation in apply element with id g_K1_calculation.
 * Equation in apply element with id g_K2_calculation.