In the "non-obviousness trilogy", the Supreme Court further developed positive secondary of non/obviousness in addition to the Graham factors as negative secondary considerations, but noted that the secondary factors could not overcome a strong "prima facie" case of obviousness. In contrast to "United States v. Adams" , where the Court recognized that when a patent claims a structure already known in the prior art that is altered by the mere substitution of one element for another known in the field, the combination must do more than yield a predictable result, In "Anderson's-Black Rock, Inc. v. Pavement Salvage Co." (1969), the two pre-existing elements in combination did no more than they would have accomplished in separate, sequential operation. Similarly, in "Sakraida v. Ag Pro Inc." (1976), a unanimous US Supreme Court found a claimed invention (a water flush system to remove cow manure from the floor of a dairy barn) obvious because it "simply arranges old elements without "producing a new or different function", although "doubtless a matter of great convenience, producing a desired result in a cheaper and faster way, and enjoying commercial success." This is an important precedent because it reversed the 1st Graham's factor (commercial success). Also, in "Dann v. Johnston"(1976) the Supreme Court cautioned that "the mere existence of differences between the prior art and an invention does not establish the invention's nonobviousness. The gap between the prior art and respondent's system is simply not so great as to render the system nonobvious to one reasonably skilled in the art." During this period, other courts have considered additional secondary considerations as well (in the order on importance according to Judge Learned Hand:  1) long felt need for a solution to a real problem which has been recognized in the prior art or in the industry. 2) failures of others to solve the same problem (while equipped with the same knowledge)  3) significant efforts and high cost of developing the invention. 4) skepticism or disbelief before the invention as an indicator of non-obviousness. 5) copying, praise, unexpected results, and industry acceptance as indicators of non obviousness. 6) copying as an indicator of non-obviousness  7) commercial success. 8) unexpected advantages. The presence of several favorable considerations make the case for non-obviousness stronger. Another important result of Graham was the decision that the non-obviousness of a patent claim is a question of law rather than a question of fact, i.e. it must be decided by a judge rather than by a jury.