a sample holder guided by said carriage for movement relative to said carriage in a direction perpendicular to the friction surface and said first direction, said sample holder being configured to hold a sample in frictional engagement with the friction surface during {HEAD ~ relative movement} between said carriage and friction surface in said {TAIL ~ first direction} 
a sample holder guided by said carriage for movement relative to said carriage in a direction perpendicular to the friction surface and said first direction , said sample holder being configured to hold a sample in frictional engagement with the friction surface during { HEAD ~ relative movement } between said carriage and friction surface in said { TAIL ~ first direction }
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