Uhrhahn Construction prepared proposals for work on construction projects for Lamar and Joan Hopkins. The Hopkinses signed the proposals for each project under sections titled "Acceptance of Proposal." The proposals stated that any changes to the written estimates and specifications had to be in writing. During the construction, the Hopkinses made several oral requests for additional work not included in the proposals. They paid for some of them after receiving invoices labeled change orders for such work. When they did not pay for the rest, Uhrhahn sued, alleging there was an implied contract to make oral changes. Was there such an implied contract? Justify your position.