func (m *BuilderMode) Set(s string) error {
	var mode BuilderMode
	for _, c := range s {
		switch c {
		case 'D':
			mode |= GlobalDebug
		case 'P':
			mode |= PrintPackages
		case 'F':
			mode |= PrintFunctions
		case 'S':
			mode |= LogSource | BuildSerially
		case 'C':
			mode |= SanityCheckFunctions
		case 'N':
			mode |= NaiveForm
		case 'L':
			mode |= BuildSerially
		default:
			return fmt.Errorf("unknown BuilderMode option: %q", c)
		}
	}
	*m = mode
	return nil
}
Set parses the flag characters in s and updates *m.
package ssa
import (
	"bytes"
	"fmt"
)
type BuilderMode uint
const (
	PrintPackages        BuilderMode = 1 << iota  
	PrintFunctions                                
	LogSource                                     
	SanityCheckFunctions                          
	NaiveForm                                     
	BuildSerially                                 
	GlobalDebug                                   
	BareInits                                     
)
const BuilderModeDoc = `Options controlling the SSA builder.
The value is a sequence of zero or more of these letters:
C	perform sanity [C]hecking of the SSA form.
D	include [D]ebug info for every function.
P	print [P]ackage inventory.
F	print [F]unction SSA code.
S	log [S]ource locations as SSA builder progresses.
L	build distinct packages seria[L]ly instead of in parallel.
N	build [N]aive SSA form: don't replace local loads/stores with registers.
I	build bare [I]nit functions: no init guards or calls to dependent inits.
`
func (m BuilderMode) String() string {
	var buf bytes.Buffer
	if m&GlobalDebug != 0 {
		buf.WriteByte('D')
	}
	if m&PrintPackages != 0 {
		buf.WriteByte('P')
	}
	if m&PrintFunctions != 0 {
		buf.WriteByte('F')
	}
	if m&LogSource != 0 {
		buf.WriteByte('S')
	}
	if m&SanityCheckFunctions != 0 {
		buf.WriteByte('C')
	}
	if m&NaiveForm != 0 {
		buf.WriteByte('N')
	}
	if m&BuildSerially != 0 {
		buf.WriteByte('L')
	}
	return buf.String()
}