Expr can be a function, expression, or equation. If Expr is not an equation, it will be assumed to be equal to zero (0). 

ImplicitVar is the implicit variable. 

ExplicitVarOrList is the explicit variable or a list of one or more explicit variables in an expression of three of more variables. 

Return value: 
The implicit derivative or a list of implicit derivatives if ExplicitVarOrList is a list of two or more variables. 

Examples: 
1. di(y=(2x+5)^-1/2,y,x,1)  (y is an implicit function of x, first order)
     -(2x+5)^-3/2 

2. di(y²=x²+2x,y,x,2)  (y is an implicit function of x, second order)
     (-x²-2x+y²-1)/y³

3. di(x²+y²+z²=a²,z,{x,y},1)  (z is an implicit function of x and y, first order)
     (-x/z, -y/z}