opa ast.Module unable to unmarshal json.(problem with some keyword)

[gururajarao79]

Iam trying to unMarshal AST json to ast.Module structure of OPA.
policy.rego:

package example.authz
import rego.v1

allow if {
    some i
    input.users[i].role == "admin"
}

use command opa parse --v1-compatible --format json policy.rego generated ast:

{
  "package": {
    "path": [
      {
        "type": "var",
        "value": "data"
      },
      {
        "type": "string",
        "value": "example"
      },
      {
        "type": "string",
        "value": "authz"
      }
    ]
  },
  "imports": [
    {
      "path": {
        "type": "ref",
        "value": [
          {
            "type": "var",
            "value": "rego"
          },
          {
            "type": "string",
            "value": "v1"
          }
        ]
      }
    }
  ],
  "rules": [
    {
      "body": [
        {
          "index": 0,
          "terms": {
            "symbols": [
              {
                "type": "var",
                "value": "i"
              }
            ]
          }
        },
        {
          "index": 1,
          "terms": [
            {
              "type": "ref",
              "value": [
                {
                  "type": "var",
                  "value": "equal"
                }
              ]
            },
            {
              "type": "ref",
              "value": [
                {
                  "type": "var",
                  "value": "input"
                },
                {
                  "type": "string",
                  "value": "users"
                },
                {
                  "type": "var",
                  "value": "i"
                },
                {
                  "type": "string",
                  "value": "role"
                }
              ]
            },
            {
              "type": "string",
              "value": "admin"
            }
          ]
        }
      ],
      "head": {
        "name": "allow",
        "value": {
          "type": "boolean",
          "value": true
        },
        "ref": [
          {
            "type": "var",
            "value": "allow"
          }
        ]
      }
    }
  ]
}

go code converting json bytes into ast.Module

  var module ast.Module
        module.SetRegoVersion(ast.RegoV1)
        if err = module.UnmarshalJSON(b); err != nil {
                panic(err)
        }

I am getting error panic: ast: unable to unmarshal term. The problem is with keyword "some". if there is "some" keyword in original rego it is not Unmarshalling to ast.Module