How to use the targ.genTempl.Error function in targ

def genArg(a) :
    """Generate a list of arg values for this argument generator."""
    if a.startswith('0x') :	# hex numbers
        return [Num(int(a[2:], 16))]
    elif a.startswith('0') and a != '0' : # octal numbers
        return [Num(int(a[1:], 8))]
    elif a.isdigit() : 		# decimal numbers
        return [Num(int(a, 10))]
    elif a.startswith('32[') :	# vec32
        if a[-1] != ']' :
            Error("bad vec arg %r" % a)
        vec = a[3:-1].split(',')
        return [Vec32(*xs) for xs in genArgs(vec)]
    elif a[0] == '[' :		# vec64
        if a[-1] != ']' :
            Error("bad vec arg %r" % a)
        vec = a[1:-1].split(',')
        return [Vec64(*xs) for xs in genArgs(vec)]
    elif a[0] == '{' :          # union of several other arg types
        if a[-1] != '}' :
            Error("bad union arg %r" % a)
        its = a[1:-1].split(';')
        res = []
        for it in its :
            res += genArg(it)
        return res
    elif a[0] == '"' :		# string literal as buffer
        if a[-1] != '"' :
            Error("bad string arg %r" % a)
	return [StringZ(a[1:-1])]
    elif a == 'fd' :		# file descriptor options
	# stdfile0 = "/", stdfile1 = tcp socket

elif a[0] == '[' :		# vec64
        if a[-1] != ']' :
            Error("bad vec arg %r" % a)
        vec = a[1:-1].split(',')
        return [Vec64(*xs) for xs in genArgs(vec)]
    elif a[0] == '{' :          # union of several other arg types
        if a[-1] != '}' :
            Error("bad union arg %r" % a)
        its = a[1:-1].split(';')
        res = []
        for it in its :
            res += genArg(it)
        return res
    elif a[0] == '"' :		# string literal as buffer
        if a[-1] != '"' :
            Error("bad string arg %r" % a)
	return [StringZ(a[1:-1])]
    elif a == 'fd' :		# file descriptor options
	# stdfile0 = "/", stdfile1 = tcp socket
        return [Num(1), File("stuffhere"), StdFile(0), StdFile(1)]
    elif a == 'fn' :		# filename options
        return [StringZ("/tmp/file9"), StringZ("/"), StringZ("/tmp"), Filename("#!/bin/sh\necho hi\n")]
    elif a == 'str' :		# initialized string/buffer options
        return [StringZ("testing"), Alloc(256), Filename("stuffhere")]
    elif a == 'buf' :		# alloc uninitialized buffer options
        return [Alloc(128), Alloc(4096)]
    elif a == 'sz' :		# sizes
        return [Len()]
    elif a == 'pid' :		# proc id options
        return [Num(0), Num(0xffffffffffffffff), ChildPid, MyPid] # skip PPid
    else :
        raise Error("bad arg type %r" % a)