## Arbitrary mathematical operations with trace headers

*call	hdrmath

eval	int c_line = s_line + r_line - 3000		
eval	int c_stnum = s_stnum + r_stnum
eval	int c_num = 10000*c_line + c_stnum

eval	real cdp_x = 0.5*(sou_x	+ rec_x)
eval	real cdp_y = 0.5*(sou_y	+ rec_y)
eval	real celev = 0.5*(selev	+ relev)