add distance before motion modifier

```
point1 := P[1]
pin1 := DO[1]

point1.joints -> [0,0,0,0,0,0]

linear_move.to(point1).at(100, 'mm/s').term(0).distance_before(100, foo())
linear_move.to(point1).at(100, 'mm/s').term(0).distance_before(100, turn_on(pin1))
```

Author: kobbled

generators/parser.y

@@ -1,6 +1,6 @@
 class TPPlus::Parser
 token ASSIGN AT_SYM COMMENT MESSAGE WARNING JUMP IO_METHOD INPUT OUTPUT
-token NUMREG POSREG VREG SREG TIME_SEGMENT ARG UALM TOOLREG FRAMEREG
+token NUMREG POSREG VREG SREG TIME_SEGMENT DISTANCE_SEGMENT ARG UALM TOOLREG FRAMEREG
 token MOVE DOT TO DOWNTO MID AT ACC TERM OFFSET SKIP GROUP COORD 
 token MROT PTH WJNT INC BREAK RTCP FPLIN
 token AP_LD RT_LD CD CR INDEV EV PSPD CTV
@@ -429,8 +429,10 @@ rule
                                        { result = TerminationNode.new(val[2],val[4],nil) }
     | DOT swallow_newlines OFFSET LPAREN var_or_indirect RPAREN
                                        { result = OffsetNode.new(val[2],val[4]) }
-    | DOT swallow_newlines TIME_SEGMENT LPAREN time COMMA time_seg_actions RPAREN
+    | DOT swallow_newlines TIME_SEGMENT LPAREN time COMMA seg_actions RPAREN
                                        { result = TimeNode.new(val[2],val[4],val[6]) }
+    | DOT swallow_newlines DISTANCE_SEGMENT LPAREN distance COMMA seg_actions RPAREN
+                                       { result = DistanceNode.new(val[2],val[4],val[6]) }
     | DOT swallow_newlines SKIP LPAREN label optional_lpos_arg RPAREN
                                        { result = SkipNode.new(val[4],val[5]) }
     | DOT swallow_newlines valid_motion_statements
@@ -486,7 +488,7 @@ rule
     |                 { result = nil }
     ;
 
-  time_seg_actions
+  seg_actions
     : program_call
     | io_method
     ;
@@ -495,6 +497,10 @@ rule
     : var
     | number
     ;
+  
+  distance
+    : number
+    ;
 
   speed
     : indirectable COMMA STRING          { result = { speed: val[0], units: val[2] } }

lib/tp_plus.rb

@@ -30,6 +30,7 @@ module TPPlus
 require_relative 'tp_plus/nodes/conditional_block_node'
 require_relative 'tp_plus/nodes/definition_node'
 require_relative 'tp_plus/nodes/digit_node'
+require_relative 'tp_plus/nodes/distance_node'
 require_relative 'tp_plus/nodes/empty_stmt_node'
 require_relative 'tp_plus/nodes/eval_node'
 require_relative 'tp_plus/nodes/expression_node'

lib/tp_plus/nodes/distance_node.rb

@@ -0,0 +1,22 @@
+module TPPlus
+  module Nodes
+    class DistanceNode < BaseNode
+      def initialize(type, distance, action)
+        @type = type
+        @distance = distance
+        @action = action
+      end
+
+      def type
+        case @type.downcase
+        when "distance_before"
+          "DB"
+        end
+      end
+
+      def eval(context)
+        "#{type} #{@distance.eval(context,as_string: true)}mm,#{@action.eval(context)}"
+      end
+    end
+  end
+end