An assembly able to be positioned in a stomach (A) to define a sleeve (G) receiving food and a bypass portion (F), the assembly comprising: a bougie configured to enter the stomach (A), the bougie comprising: an elongated body having a segment being shaped to follow a lesser curve (C) of the stomach (A), and an extension member having a proximal portion connected to the segment of the elongate body , the extension member being movable from a retracted position in which the extension member is generally aligned with the elongate body , to an extended position in which the extension member extends at an angle away from the segment within the stomach (A) for positioning a distal portion of the extension member away from a gastroesophageal junction (E) of the stomach (A); and a sheath comprising a tubular body having an elongated main lumen in which the bougie is inserted, the sheath being translatable along the bougie between: a first position in which the tubular body at least covers the extension member of the bougie for maintaining the extension member in the retracted position, and a second position in which the tubular body is staggered with respect to the extension member for allowing the extension member to move from the retracted position into the extended position. a bougie configured to enter the stomach (A), the bougie comprising: an elongated body having a segment being shaped to follow a lesser curve (C) of the stomach (A), and an extension member having a proximal portion connected to the segment of the elongate body , the extension member being movable from a retracted position in which the extension member is generally aligned with the elongate body , to an extended position in which the extension member extends at an angle away from the segment within the stomach (A) for positioning a distal portion of the extension member away from a gastroesophageal junction (E) of the stomach (A); and an elongated body having a segment being shaped to follow a lesser curve (C) of the stomach (A), and an extension member having a proximal portion connected to the segment of the elongate body , the extension member being movable from a retracted position in which the extension member is generally aligned with the elongate body , to an extended position in which the extension member extends at an angle away from the segment within the stomach (A) for positioning a distal portion of the extension member away from a gastroesophageal junction (E) of the stomach (A); and a sheath comprising a tubular body having an elongated main lumen in which the bougie is inserted, the sheath being translatable along the bougie between: a first position in which the tubular body at least covers the extension member of the bougie for maintaining the extension member in the retracted position, and a second position in which the tubular body is staggered with respect to the extension member for allowing the extension member to move from the retracted position into the extended position. a first position in which the tubular body at least covers the extension member of the bougie for maintaining the extension member in the retracted position, and a second position in which the tubular body is staggered with respect to the extension member for allowing the extension member to move from the retracted position into the extended position. <EOS>