diff --git a/grow/definition.go b/grow/definition.go
index 0861415..c35ba94 100644
--- a/grow/definition.go
+++ b/grow/definition.go
@@ -7,10 +7,14 @@ import "gopkg.in/yaml.v2"
 import "github.com/firestuff/subcoding/vm"
 
 type Definition struct {
-	GlobalMemorySize   uint64    `yaml:"global_memory_size"`
-	FunctionMemorySize uint64    `yaml:"function_memory_size"`
-	InstructionLimit   uint64    `yaml:"instruction_limit"`
-	Samples            []*Sample `yaml:"samples"`
+	GlobalMemorySize   uint64 `yaml:"global_memory_size"`
+	FunctionMemorySize uint64 `yaml:"function_memory_size"`
+	InstructionLimit   uint64 `yaml:"instruction_limit"`
+
+	InstructionsPerFunctionMean   uint64 `yaml:"instructions_per_function_mean"`
+	InstructionsPerFunctionStdDev uint64 `yaml:"instructions_per_function_std_dev"`
+
+	Samples []*Sample `yaml:"samples"`
 }
 
 func NewDefinition(r io.Reader) (*Definition, error) {
diff --git a/grow/defs/pythag.yaml b/grow/defs/pythag.yaml
index 2ba164b..47c00bc 100644
--- a/grow/defs/pythag.yaml
+++ b/grow/defs/pythag.yaml
@@ -1,30 +1,25 @@
 global_memory_size: 2
 function_memory_size: 1
 instruction_limit: 20
+
+instructions_per_function_mean: 5
+instructions_per_function_std_dev: 3
+
 samples:
 - in:  [3,4]
   out: [5]
 
-- in:  [5,12]
-  out: [13]
-
-- in:  [8,15]
-  out: [17]
-
 - in:  [7,24]
   out: [25]
 
-- in:  [20,21]
-  out: [29]
-
-- in:  [12,35]
-  out: [37]
-
-- in:  [9, 40]
-  out: [41]
-
 - in:  [88, 105]
   out: [137]
 
+- in:  [140, 171]
+  out: [221]
+
+- in:  [105, 208]
+  out: [233]
+
 - in:  [161, 240]
   out: [289]
diff --git a/grow/mutate.go b/grow/mutate.go
index 5cde07e..d5598d0 100644
--- a/grow/mutate.go
+++ b/grow/mutate.go
@@ -5,13 +5,8 @@ import "math/rand"
 import "github.com/firestuff/subcoding/gen"
 import "github.com/firestuff/subcoding/vm"
 
-const instructionsPerFunctionMean = 15
-const instrucitonsPerFunctionStdDev = 10
-
-const minifyPeriodMean = 10000
-
-func Mutate(_ *Definition, prog *vm.Program) {
-	target := int(rand.NormFloat64()*instrucitonsPerFunctionStdDev + instructionsPerFunctionMean)
+func Mutate(def *Definition, prog *vm.Program) {
+	target := int(rand.NormFloat64()*float64(def.InstructionsPerFunctionStdDev) + float64(def.InstructionsPerFunctionMean))
 
 	if len(prog.Functions[0].Instructions) < target {
 		addInstruction(prog, prog.Functions[0])