A 38-year-old man, an arc welder, is a chain smoker. He came to the dentist to have a denture made for him. Howerver, in the middle of his lower lip against the background of unchanged vermillion border there is a gray-white polygonal lesion 6x4 mm with clear margins. The lesion is covered with tightly attached small scales and is level with the vermillion border (neither protruding nor retracted). Palpation of the lesion focus is painless, the lesion has no thickened base; lateral palpation detects thickened surface of the lesion. Regional lymph nodes are unchanged. Make the provisional diagnosis: