Expr can be a function, expression, equation, or inequality. If Expr is not an equation, it will be assumed to be equal to DependantVar. 

DependantVar will become the independant variable in the returned expression. 

IndependantVar will become the dependant variable in the returned expression. 

Return value: 
The inverted expression with dependant and independant variables swapped. 

Examples: 
1. inv(y=2x+4,y,x) 
    y=x/2-2 
 
2. y=x²+2x-4,y,x) 
     y={(x+5)^1/2-1,(x+5)^1/2+1}