(expr) ---> (5 * _1)
(eval( expr, x_=5)) ---> 25
-------------
 ------ start  test2  -----------------
(expr) ---> (_1 + (2 * _2))
(eval(expr,x_ =1, y_ =2)) ---> 5
(eval(expr,x_ =1)) ---> (1 + (2 * _2))
(eval(expr,x_ =x_ + y_)) ---> ((_1 + _2) + (2 * _2))
(eval( eval ( expr,x_ =x_ + y_), x_ = 1, y_ = 2)) ---> 7
 ------ start  test2  -----------------
(expr) ---> ((_1 + (2 * _2)) + _1)
(eval(expr,x_ =1, y_ =2)) ---> 6
(eval(expr,x_ =1)) ---> ((1 + (2 * _2)) + 1)
(eval(expr,x_ =x_ + y_)) ---> (((_1 + _2) + (2 * _2)) + (_1 + _2))
(eval( eval ( expr,x_ =x_ + y_), x_ = 1, y_ = 2)) ---> 10
 ------ start  test2  -----------------
(expr) ---> ((_1 / 2) + (2 * _2))
(eval(expr,x_ =1, y_ =2)) ---> 4.5
(eval(expr,x_ =1)) ---> (0.5 + (2 * _2))
(eval(expr,x_ =x_ + y_)) ---> (((_1 + _2) / 2) + (2 * _2))
(eval( eval ( expr,x_ =x_ + y_), x_ = 1, y_ = 2)) ---> 5.5
 ------ start  test2  -----------------
(expr) ---> F1(_1)
(eval(expr,x_ =1, y_ =2)) ---> 10
(eval(expr,x_ =1)) ---> 10
(eval(expr,x_ =x_ + y_)) ---> F1((_1 + _2))
(eval( eval ( expr,x_ =x_ + y_), x_ = 1, y_ = 2)) ---> 30
 ------ start  test2  -----------------
(expr) ---> (F1(_1) + (2 * _2))
(eval(expr,x_ =1, y_ =2)) ---> 14
(eval(expr,x_ =1)) ---> (10 + (2 * _2))
(eval(expr,x_ =x_ + y_)) ---> (F1((_1 + _2)) + (2 * _2))
(eval( eval ( expr,x_ =x_ + y_), x_ = 1, y_ = 2)) ---> 34
 ------ start  test2  -----------------
(expr) ---> ((1 / F1(_1)) + (2 * _2))
(eval(expr,x_ =1, y_ =2)) ---> 4.1
(eval(expr,x_ =1)) ---> (0.1 + (2 * _2))
(eval(expr,x_ =x_ + y_)) ---> ((1 / F1((_1 + _2))) + (2 * _2))
(eval( eval ( expr,x_ =x_ + y_), x_ = 1, y_ = 2)) ---> 4.03333
4 = 4
12 = 12
9 = 9
9 = 9
12 = 12
-------------
 f.v before assign 7 
 f.v after assign 8 
-------------
 fb.v before assign 7 7
 fb.v after assign 7 8
-------------
expr = (F2(_1, _2) + (2 * _2))
eval(expr,x_ =1, y_ =2) =  16 and it should be 16
 tmp =F2(2, _2)
another  =  F2(2, _2)
another  =  12
-------------
(eval( if_else( true , 2*x_ , y_) , x_=1, y_=3)) ---> 2
(eval( if_else( false , 2*x_ , y_) ,x_=1, y_=3)) ---> 3
(eval( if_else( x_>y_ , 2*x_ , y_) ,x_=1, y_=3)) ---> 3
(_1 < _2)