Measurement of the production cross-section of positive pions in p–Al collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:mn>12.9</mml:mn><mml:mtext> GeV</mml:mtext><mml:mo stretchy="false">/</mml:mo><mml:mi>c</mml:mi></mml:math>