### 11.18.2 Envelopes: envelope

The envelope command takes two arguments, the first argument is expression
expr in the variables x, y and a parameter
such as t, and the second argument is the name of the
parameter.

envelope draws the envelope of the family of curves given by
expr=0 over the parameter t.

Input:

envelope(y + x*tan(t) - 2*sin(t),t)

Output:

The envelope command can use variables besides x and y. In
this case, the second argument needs to be a list of the two variables
and the parameter.

Input:

envelope(v + u*tan(s) - 2*sin(s),[u,v,s])

Output: