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functions.h
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#ifndef H_OT_FUNCTIONS
#define H_OT_FUNCTIONS

extern float lin_interp(float pleth, float x[], float z[], int istart, int npts);

#endif
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functions.c
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#include "functions.h"

float lin_interp(float pleth, float x[], float z[], int istart, int npts)
{
...
return whatever;
]
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